National Institute of Mental Health

NIMH (National Institute of Mental Health) were the central antagonists of the entire Secret of NIMH series and were based very loosely on a real-world organization of the same name, however the name is where the similarities end and the fictional NIMH is considered (at least by animals) as one of the most evil places in the world.

NIMH was responsible for the creation of the hyper-intelligent rats that would eventually become known as the "Rats of NIMH" and it was these rats that orchestrated a daring escape with the aid of several mice - having had enough of the continued abuse NIMH regularly put them through as part of medical experiments that were clearly not monitired correctly (for example they actively captured wild rodents for experimentation, a practice that is no longer widely practiced due to animal welfare (as well as the fact most reputable labs breed their own mice / rats) ).

NIMH is a faceless evil and is often mentioned rather than actually faced, though in the sequel NIMH would play a bigger role and arguably become the main antagonists of the film - in fact if one were to be picky one could argue NIMH was also the main antagonist of the original film as they had created Jenner (though, somewhat ironically, they also created Nicodemus and other "good" rats - who aided Mrs. Brisby in her quest).

It is unknown what became of NIMH after the series ended but it is presumably still active, whether or not it has improved its (to be frank) horrific treatment of animals is also left unknown..