The common wisdom in introducing a newly adopted cat to a resident one in the past was to open the carrying case and "let them work it out." We most definitely have a new way of looking at things; from the cat's perspective. Cats are, after all, about territory. Bring a new, utterly alien scent of the same species into the house, and more times than not, we're asking for chaos. Of course everyone has a story about introducing two cats that went smoothly doing the old fashioned technique. The point to stress is, if it goes poorly, this one meeting is the association that these two cats will hold onto for quite a long time and make a peaceable kingdom a difficult task. It is, ultimately, better to be safe than sorry.
A slow and steady introduction starts with the establishment of a base camp for the newcomer. Once you've set up his or her space, you're ready to start letting the cats make positive associations between one another. This is key, and will be repeated ad infinitum; all associations between the cats during this critical period have to be as pleasing as possible to reduce possible friction when they finally have free access.
Let's start with one of the most pleasing motivators-food! Feeding time will happen at the door of base camp until introduction is complete. If the resident cat is not on a scheduled feeding diet, it might be best to put him or her on one for now. Or, if you leave dry food out and supplement with wet food, greatly decrease the amount of dry so that wet feeding time is looked forward to more. Remember that the only time either cat gets wet food is during these "meet and greets" at the base camp door, which can be divided into two daily sessions. Place food bowls on either side of the door with a couple of feet of breathing room for each cat. Ideally, there should be a family member on either side of the door to praise each cat as they eat. The idea is that they are rewarded with food for being so close to the scent of the unfamiliar cat, and also rewarded by you with praise for eating. At this initial point, the door should be closed; the cats can smell one another just fine. If they don't devour their food at first, that's okay. They will eventually eat. Don't give in and move the food.
The next step is to open the door just a tiny crack, giving the cats limited visual access to each other. How soon do you move on to this step? As with all steps in introduction, pay attention to the cats; let their body language tell you when they are comfortable enough to move on. Remember that proceeding too quickly will force you to jump backwards by anywhere from a few days to a few weeks. Slow and steady definitely wins this race. We need to remain safe, so use rubber doorstops on either side of the introduction door to prevent any more than visual accessibility. If the door is too high off the ground to use stops, or if one or both cats are muscling the door open, try using a hook and eye setup. Instead of using it to lock a door shut, you would employ it backwards, to give us just a couple of inches of cracked space between the door and the jamb.
Again, the time required in moving from step to step is determined by your observation and the cats' level of comfort. Keep cracking the door further until each cat could, if they wished, bat at one another-first up to the elbow joint then all the way to the shoulder, just making sure not to leave enough room to let a head get through. The object of "the game" is to give them enough rope to succeed. If they fail, just go back to the previous step.
Other tricks to use during the introduction period are "scent swapping" and "site swapping." In scent swapping, we take a washcloth per cat and rub them down with it, making sure to go across their cheeks, head, sides, and around the base of the tail. Then, present the other cat with the scent of the washcloth in a conspicuous part of their territory, perhaps near a favored sleeping spot or near (but respecting the space of) their food or water. This will start getting them accustomed to the new facts of life; their space will have to be shared with one another, and better to have this fact introduced by scent than sight.
Site swapping relies on more paws-on physical exploration of one another's space. Once a day, switch the two cats. The new cat gets to explore the house while the resident cat is base camp to freely explore the scent of new arrival without the fear of retribution. This process is best done with a human partner just to make sure the cats don't inadvertently get in each other's way while trading places; but if you don't have help, try putting the resident in, say, a bedroom. When the new cat heads for the kitchen or other area out of sight, move the resident cat into base camp. Both cats should get the praise and encouragement they need/deserve in bravely going where they have not gone before!
Don't forget, during this entire process, to play with the cats! This may seem elementary, but remember, they are just energetic balloons naturally, and even more so during these intense times of stress. Of course, you will have separate play sessions during the introduction phase. Once they've met and cohabitated for a bit, group playtime will be another wonderful way of diverting aggression they might have towards one another into a positive route.
When you think it's time to let them be in the territory together at the same time, take precautions. If a fight breaks out, do not try to break it up with your hands! Unfortunately, this is most of the time our first instinct. You are almost sure to be clawed and bitten, and it will not be pretty. In the heat of the moment, the cats will not be able to distinguish between your arm and each other, and they will have no inhibition about attacking whatever is handy, even if it's you. Instead, have an immediate barrier like a couple of large, thick towels or blankets at the ready. You can toss them over the cats to disorient them, and immediately relocate them by scooping them up inside the towel (to protect yourself). There is no need to follow this up with a scolding. That will not do anything except increase the cats' agitation, which is just what you don't need! Let the event pass with each cat in their own "time-out", and start again fresh tomorrow-at the very beginning. Also make sure that when the two cats meet, they have escape routes from one another. Getting cornered is a sure recipe for a fight in the mind of a defense-minded animal like a cat.
Keep a close eye on all interactions for the first week or so, not letting the cats have free access to one another when nobody is home. Finally, keep the food and litter setup established in the base camp room, at least for the next while. The accepted "recipe" is three litterboxes for two cats (to be precise, 1 box per cat + 1), so bear that in mind. Also bear in mind escape routes from the boxes, as the last place we want a skirmish to erupt is while one of the cats is having a "private moment." They should be able to see as much of the room around them as possible when in the litterbox, which is why uncovered boxes would be highly recommended.
This should pretty well cover the bases for the initial introduction between your cats. Of course there are always variables, but the broken record theme should get you going; do it slow—there’s always tomorrow to make another positive impression. They can, over time, learn that every time they view or smell the other, something good will happen. Do it too quickly and that negative first impression might very well be the one that lasts.
By Jackson Galaxy