Mrs. Frisby and the Rats of NIMH/The Secret of NIMH
There are three significant differences between the original novel and the movie version of this story.
1. In the book, Jenner is never actually seen. He has already left the colony and we hear the strange story of some dead rats, presumed to be Jenner and his group. This serves as a cautionary tale. In the movie, Jenner plays an integral role. He actively conspires against the others and even murders one in a quest for power making the movie version much darker.
2. In the book, the tests done by NIMH were specifically designed to test and increase the intellectual capacity of the rats. They escape out of a natural desire for freedom. In the movie the tests are all very cruel and almost haphazard. Their resulting intelligence seems to be a random side effect.
3. In the book, the rats use their knowledge of mechanics to create their lair, move to their new home, and move Mrs. Frisby's home. Science saves the day. The movie adds a great deal of, for lack of a better term, magic. Everything hinges on a mystical amulet that lifts the Brisby's home from the mud and which is powered by a courageous heart.
The other changes are mostly for cinematic effect (reducing the rats' story and plan, making Jeremy a comedic figure to counter balance the darkness of the plot). The one that confuses me the most is the name change. The movie makes the name Mrs. Brisby. Its a minor change but entirely unnecessary and a little disconcerting.