Mrs. Frisby and the Rats of NIMH
by Robert C. O'Brien
Newbery Medal Winner (1972)
Genetically altered vermin developing their own agricultural society-- Run! Run for the hills! Actually no, stay away from the hills, since that's where the genius immortal super-sized rats are!
Mrs. Frisby is a widowed country mouse with a sick mouse-child who discovers the neighborhood rats are not all they seem; after chemical/brain testing by scientists the rats (and a few mice) escape the lab and learn how to use electricity. Some rats come to somewhat gruesome ends, but most of the story is positive and entertaining.
Did they ever tell us what "NIMH" stood for? And is there a sequel?