Instructions
Start by putting each rat in the other's cage for a couple of hours before introducing them. This helps them adjust to the other's smell.
Place rats in cages side by side (not so close that they can reach out and hurt one another) so that they become familiar with each other at a safe distance.
Introduce them on neutral territory, such as a brand new cage or a closed off bathroom. Make sure you don't leave them unattended during this introductory phase. Keep the meetings brief, maybe 15 minutes at the most.
Invest in a brand new cage that has no chance of harboring any remnants of existing rat odor. Even if you feel sure you have cleaned it spotless, a previously used cage is sure to affect the relationship of the rats.
Feel comfortable that the tension has subdued? When they are ready to be caged together, put in a new nesting box, small jar or piece of PVC pipe for the new rat to hide in.
How to Introduce a New Pet Rat to Another Rat
Need to introduce a new rat to another or an existing group? Whether you have cage mates that have been apart for awhile, babies or an older rat, adding a new companion can cause problems as the new rat finds its place in the social structure. There are times that rats will simply clash and will not get along. If, despite trying these tips, the introduction doesn't work, you may need to consider keeping those rats in separate cages permanently. Good luck!