Carry the new cat into the house in her carrier. If someone other than the people who live in the house bring in the new cat, the old cats are less likely to hold their guardians responsible for invasion of their territory.
A safe haven
Take the new cat, still in the carrier, into a separate room and close the door. Let the new cat out and help her get accustomed to the room. She’ll be there for a few days, so provide food, water, toys, a nice place to sleep, and a litter box (place the litter box in a corner of the room across from doorways, away from the food and water dishes).
Encourage the new cat to play or eat close to the door. With a friend or relative on the other side of the door, encourage the old cats to play or eat close to the door, as well. When feeding, don’t put the food so close to the door that the animals are too upset by each other’s presence to eat. Gradually move the dishes closer to the door until your pets can eat calmly while standing directly on either side of the door. Also, consider serving a special treat like wet food, so that the old cats associate the stranger with positive feelings. A cat’s natural inclination is to protect their territory by running the intruder out. The easiest and fastest way to counter that inclination is with rewards: food, play, treats, catnip, praise, stroking, anything they like. If they learn to associate good things with the intruder, they won’t want to drive him or her away.
The old switcheroo
Cats strongly rely on their sense of smell, so they need to get accustomed to each other’s scent. Exchange sleeping blankets, brush all the cats with one brush, and pet the new cat with a towel, then put it under the old cats’ food dishes (and vice-versa). After this step is successful, swap the cats’ position by allowing the old cats in the safe room and the new cat to roam the house. Make sure you switch the litter boxes, as well, so the new cat is always using her box (it is advised that separate litter boxes be used for as long as needed, perhaps always). You may find that the cats are starting to play with each other under the door. You can encourage this by sliding a toy under one side and wiggling it around, so both cats can see it. Once the cats are playing together, quietly leave them to it. Do not move on to the next step until at least a week has past.
When you are ready for the first face-to-face introduction, put the new cat in his carrier and let the resident cats come into the safe room. This will give you an opportunity to observe the interaction among the cats while the new cat is protected in his carrier. After 10 minutes or so of rewards, take the old cats out.
Come out, come out, wherever you are
To test whether it’s safe to let the newcomer out of his room with the other cats, situate an old cat, preferably the alpha cat, on the other side of the door. Open the door a crack and see what happens. If there’s much yowling, hissing, or spitting, gently close the door and keep up the carrier and under-door rewarding routine. When the two cats seem comfortable with limited exposure, try feeding them on opposite sides of the same room, then return them to their separate quarters. After a few days of common mealtimes, they may be ready to share the same living space.
Let the cats mingle
When the cats seem to be comfortable with the routine, let the newcomer out. If you already have more than one cat, use the “alpha cat” for preliminary introductions. Once she accepts the newcomer, the other resident cats will quickly fall in line. Supervise these initial visits carefully. Heap love, praise, play, whatever, onto all cats, but especially remember to praise the old cats. It’s their space that’s being violated, so help them feel comfortable with the newcomer. Let them know that nothing will change for them: they’ll still get as much food, love, attention, play as they did before. Reward them whenever the newcomer is near. Slowly increase the amount of time the newcomer is out and about, until he or she is out all the time. Make their first activities together enjoyable ones so they will learn to associate pleasure with the presence of the other cat. Feeding (with their own separate dishes), playing, and petting. Keep up with the praise!
Slow and steady wins the race
Ignore hissing and growling, but you may have to intervene if a physical battle breaks out. Lift the new cat into your arms and carry him into safe territory, while your spouse/mother/sister/cousin/friend reassures the old cat that they’re still loved. If the newcomer looks to be picking a fight, warn him off. Newcomers know they’re on someone else’s turf, and so tend to be cautious. Verbally reinforce this. If they do seem to tolerate each other, even begrudgingly, praise both of them profusely. Do not leave the cats alone unsupervised until you are comfortable that there will not be aggressive behavior displayed by any of the cats. During the first few weeks, the new cat should stay in his “safe room” when no one is home to supervise. If interaction among the cats deteriorates instead of improving, return the new cat to his “safe room.” At this point you will need to start the introduction process again, this time, taking more time at each stage.
A spoonful of sugar
Some cats are less sociable toward others and less willing to share their territory. If the cats are consistently aggressive toward each other, it may be necessary to medicate one or both cats. A medication like Prozac should be prescribed by your veterinarian, and amazingly, within a week, it can revolutionize the relationships in the house – with no serious side effects. Both cats are likely to experience some anxiety during the transition period, though one cat will likely react more strongly than the other. Unless there is a medical reason not to use medication or one cat seems completely unbothered by the other, both cats are likely to benefit from medication.
Patience is a virtue
Do not rush this process. It is very important to the long-term harmony of the cats’ relationship that the introduction process proceeds at a pace comfortable for each of the cats. The introduction can take from two hours to six months, so don’t be discouraged if your cats don’t seem to get along well at first. Often the case is that they will eventually be “best buddies.” With patience and perseverance, you can turn what might appear at first as an “armed camp” into a haven of peace for your integrated feline family. Congratulations on opening your heart and home to another cat in need!