1. Observe new rat closely during quarantine for any signs of a URI (upper respiratory infection), or other health concern.

2. Place cages next to one another in the same room for 2-3 days.

3. Swap rats into each others' respective cages WITHOUT letting them interact or meet face to face. Let them explore each others' cages, scent mark, and take in the smells. Do this at least once a day for 2-3 days.

4. Introduce the rats in a neutral territory where neither has access to regularly (this is commonly a bed or a bathtub). Keep introductions short and sweet - allow time to see and sniff, then remove before any tension is able to build up. Immediately after removing the rats, give each a treat they love, and put them some place familiar and safe. You can do this several times a day for another 2-3 days, gradually increasing the amount of time spent together.

5. Introduce the rats in an area commonly used by either rats (free range area). Again, observe, and go slowly. Follow instructions from step 4 for another 1-2 days.

6. COMPLETELY clean out the cage that they are both going to be living in. This means removing all bedding and toys, cleaning hammocks and fabric in hot water in the wash, and bleaching the cage. You want the cage to be as good as new, so that the resident rat does not associate the cage as his territory. When 'furniture' goes back into the cage, put it in different places than the resident rat is used to.

6. Move newly cleaned cage into a room it is not normally kept in and add rats. Observe closely and do not leave them unattended. Squeaking and screeching is normal as they figure out their hierarchy. Rats often flip a submissive rat onto it's back and 'power groom' it's belly to express dominance. The submissive rat may squeal as though he's dying, but if there is no blood being drawn, there is no need to worry.

NOTES: Gradually increase the amount of time the rats spend in the same cage. If you feel that the rats are getting along perfectly fine, it is likely safe to leave the new rat in with the resident rat(s) overnight, however go slowly, and be patient. You may need to try step 6 a couple of times before they are ok with each other in the same cage. Depending on your individual rats, you may be able to shorten the steps, or you may have to go a bit slower.
Remember it is always safer to go slower than to rush introductions. Rats that are rushed are going to be stressed, and can easily cause serious injury to one another.


All info contained in this article is found
here.