Once upon a time, people sent monkeys and dogs into the stratosphere as test subjects aboard rockets we weren't yet willing to put a human being on top of. We also used mice to gauge how microgravity was likely to affect humans, as seen in the old video clip above.
Today, animals still go into space, but for very different reasons. And NASA's top veterinarians have developed far more stringent guidelines governing how research animals in space can be treated . The rules are aimed at finding a good balance between the desire to be humane, the need to dissect most of the creatures that do go into space, and the constraints of working in a cramped environment.