"Supporters of the use of animals in experiments, such as the British Royal Society, argue that virtually every medical achievement in the 20th century relied on the use of animals in some way, with the Institute for Laboratory Animal Research of the U.S. National Academy of Sciences arguing that even sophisticated computers are unable to model interactions between molecules, cells, tissues, organs, organisms, and the environment, making animal research necessary in many areas." -Wikipedia.org
While this may be true, laboratory animals are subjected to the most unusual, painful and often times trivial experiments, and what's worse is that those animals often fall into the hands of sick, irresponsible scientists and researchers.
Links on the subject:
http://www.peta.org/issues/animals-used-for-experimentation/default.aspx (I think PETA is a bit of a dramatic organization but they've got their own opinions too.)
These are disturbing to some people. Yes, they're real so please don't continue if you've got a faint heart.