Bringing home a new pet requires a period of transition and adjustment for other pets in the household. Some pets may hide from the new addition and others might try to push it around. Pets may begin attention-seeking behavior such as barking, pawing, stealing items, pushing the new addition out of the way, and jumping. This behavior should discontinue within a week or two. If the animals in the household do not revert to normal behavior within a short time or if they become aggressive, a qualified specialist may be needed to resolve the issue.
Make sure the new pet is healthy and has up-to-date vaccinations. Test results for fecal parasites must be negative. New cats must have their viral titer (feline immunodeficiency virus [FIV], feline leukemia virus [FELV]) status checked. Cats with positive results should not be brought into a negative household.

Gradual introductions are important. The new pet should be kept separate from the other pets unless they are closely supervised. The new pet should be placed in a neutral area (den, finished basement, brightly lit bathroom) with toys, a blanket, and water. The new pet should not be placed in an area that is considered highly desirable by the other pets. Areas of high value include places where people spend a lot of time with the pets (bedrooms) or where the pets choose to stay when they are alone (around food dishes or on window sills that are good perch sites). If your dog is crated, you can accustom the dog quickly to a new dog by crating the new dog across the room where it can be seen by the original one. As the dogs become more accustomed to each other, their crates can be moved gradually closer together until they are side by side.

The area in which you are confining the new pet must be "pet-proof." Toilet seats should be down, electric cords should be tied up, sockets should be protected with child guards, and fragile items should be moved.

Whenever any animal is isolated, it is critical that the animal receive social attention whenever possible. Spend some time alone with the new pet. Then, bring the new pet outside on a leash or harness and introduce it to the other animals.

If the animals in the household are calm and ignore each other or act friendly despite the new addition, you can feed them within sight of the new pet. Feeding and petting the animals in each other's presence teaches them that good things happen when they are together and calm. For this to be successful, neither side can react violently. If a pet does react this way, banish that animal to a neutral zone immediately and try again when it is calm. If it again reacts violently, banish the pet for the rest of the day or evening and try later in the day or the next morning.

Some aggressive and undesirable interactions are not violent but are still not conducive to the development of a good relationship between the pets. You can learn to watch for subtle behaviors that can signal potential problems. This behavior includes pilo-erection (hair lifting on scruff, neck, or back), staring, snarling, stalking, side-by-side posturing with growling or lip lifting, and pinning the other animal by grabbing its neck. Cats are masters of subtle threats, including a direct stare and an elevation of the rump and base of the tail with or without pilo-erection. If you believe the new pet is losing the contest, is terrified, or is becoming too aggressive, separate the animals. Do not put your hands or other body parts between the animals.

Reward good behavior with small food treats and petting. Leashes can be tied to furniture or doorknobs that allow the pets to sniff each other and react, but do not permit them to lunge at and injure one another. Never leave a tied pet unsupervised.

Make sure each pet has 5 to 10 minutes alone with you each day. This attention can include grooming, playing with a toy, or petting. Make sure that the pet is happy and relaxed at these times. A regular schedule may reduce anxiety about the new addition because they can rely on individual attention.

Put a bell on the new animal so that you always know where it is. During this period when you are beginning to provide the pets with free access, provide additional water dishes, litter boxes, beds, and toys to minimize competition and aggressive interaction.

The keys to success are patience and observation. The social system in the household may shift. Let the animals set their own pace. In many cases the pets never become close companions but are reasonably content leading separate lives under the same roof. Do not push the animals to develop relationships they clearly do not want.

If aggression between new pets continues, you can try a behavioral modification technique called flooding. In flooding, one animal is kept confined or otherwise restrained while it is reacting inappropriately in the presence of the other animal. An aggressive animal can be crated for an extended period with food, water, toys, litter box, if necessary, and a blanket, while the other animal is locked in a room with it or placed in a similar cage facing the aggressor. If the animals become more aggressive and upset, flooding should be stopped. This technique is a last resort and should not be attempted without qualified advice.

Medication may be useful in addition to behavioral and environmental modification. In extreme cases, in which treatment has failed, the best solution may be to place one of the animals in a new home.