leaning to one side?

Question
My husband and I have a rat who is about 1.5 years old. Today when I came in to check on her she was leaning to her left side, and continued to do so even when she walked (making it harder for her to balance). Other than this however she seems to be fine. She is still eating and drinking normally and still comes to say hi. Should I be concerned, or could she perhaps have just pulled a muscle in her back?

Answer
 Most of the time this kind of off-balance behaviour is caused by an ear infection, brain tumor, or stroke.  In the case of an ear infection, treatment with antibiotics can usually reverse the behaviour and get the rat back to normal, but with the other two they will often continue to get worse until it affects their quality of life so much that they need to be put to sleep, though it isn't impossible for them to recover at least some of their balance.  I would definitely suggest getting her to a vet, who will likely prescribe some antibiotics to see if that will help.  One and a half years old isn't that old, though it's getting up there, so hopefully it is just a simple ear infection that can be treated.  Let me know how things progress; good luck with your rat!