Introducing a New Pet
You've decided it's time to add a new pet to your growing family. How is your current cat or dog going to react? What can you do to help the process go successfully? With a little planning and lots of love your newcomer and current family pet will be pals in a short time.
How well pets will get along is predominantly determined by each pet's disposition. Dominant cats or dogs may turn aggressive during initial introductions, while fearful pets become stressed and depressed if proper preparations are not made. In general, adult cats and dogs are more accepting of kittens and puppies as they are not threatening to the resident pet's status. When introducing an adult dog or cat, expect that they may have some sorting out to do to determine who is "top dog."
It's best to introduce anew pet in baby steps. First, make one room of the house off limits to your current pet, creating a home base for the new pet. If possible, introduce the new pet's scent to the resident pet before a true nose-to-nose meeting. Take a sock and pet the new animal with it. Then, leave the scented item around the house for the resident pet to find, sniff and investigate.
After a few days, have the pets switch places for a half-hour or so allowing each pet to investigate the other critter's smells up close. After that, open the door and let them meet at their own speed and terms. They may hiss or growl, but unless the situation escalates to imminent attack, let them sort things out. Interrupting too soon may actually delay the determination of who's to be the head honcho and force a replay of this behavior at a later date. Be sure to always give your pets plenty of quality time and attention.
Source: Nestle Purina PetCare Company