A lot of scenes are set in animal labs, so a mouse is forced to swim, and we see lots of upset caged monkeys. Ella, the main monkey is injected a number of times and she makes it clear she does not like it. The film includes a disclaimer up front that no animals were really harmed, but still it is tough to watch.
In fact, the director hired special trainers that love and train monkeys to make the movie, I forgot the name of the trainers but it is displayed right before the film.