It's an engaging SF medical thriller leveraging Crichton's background as an MD although grade school notions about systems engineering make parts of this silly. The animal experimentation scenes involved the real use of carbon dioxide asphyxiation to simulate death. Maybe that was ok by ASPCA standards in 1971 but that kind of suffocation is known to be terribly painful; doing it for entertainment is reprehensible.
Wise and Gidding (director and screenwriter) succesfully repeat the formula applied when adapting the psychological horror classic "The Haunting". A concise emphasis in human drama and scientific verosimilitud over a seemingly far-fetched subject. Tense and paranoid atmosphere that grows in its final stages, despite the overload of dialogue. An adrenaline-pounding thriller.
Parts of this movie deeply disturbed me when I first saw it in a theatre in 1971 at the age of 7 or 8, and the later hard science sequences put me to sleep in the cinema chair. It was rated G at the time (and still is), but it should have been PG. The scenes in the town with the dead corpses, and especially the later ones with the animal tests, were very shocking to me at that age. Still, it IS a masterpiece. ***** 5