Mrs. Frisby and the Rats of NIMH

Mrs. Frisby and the Rats of NIMH is a 1971 children's science fiction and fantasy novel by Robert C. O'Brien, illustrated by Zena Bernstein and published by Atheneum Books in Los Angeles. It won the 1972 Newbery Medal. Ten years after publication it was adapted for film as The Secret of NIMH in 1982.

The story is built on a colony of escaped laboratory rats who live in a technologically sophisticated, literate society that imitates human arrangements. They were inspired by John B. Calhoun's research into mouse and rat population dynamics at the National Institute of Mental Health between the 1940s and the 1960s, which is why the animals' intelligence is presented as an experimental consequence rather than as a fairy-tale given.

Mrs Frisby is a field mouse trying to protect her children and her home from a farmer's plough, and the rats come to her aid in a way that creates an obligation in both directions. O'Brien died in 1973, and his daughter Jane Leslie Conly wrote two sequels. The book is often described as an adventure because the plot turns on a rescue, but its reputation rests on the arrangement between the ordinary mouse and the rats who are outside the natural order.