I remember when I was a kid saw my first horror movies in the late 70s, this one scared me to death. It's full of atmosphere, like if you know John Carpenters "Thing", but there's no Monster or Alien which kill one after another, the horror is more like "real life". The loneliness of the place where these scientists are working with the apes is more than claustrophobic and the more you stand by the cast and going through that story the more mad you'll get. The score give it's parts perfectly to it, old 70s synthesizer - sounds bring mad and scary atmosphere. I really hope this will find it's way to DVD ! A real underrated classic !