The Rats of NIMH are a colony of escaped lab rats made intelligent by some experiments at NIMH. These experiments made them at least as intelligent as humans, and, as a side effect, made them live longer (also about as long as humans) as well. Ironically, by making them intelligent, they were able to figure out how to escape.
The group is led by a rat named Nicodemus and reside in a place called Thorn Valley.
They are the subject of Robert C. O'Brien's book, Mrs. Frisby and the Rats of NIMH.