Toxicologists like myself make a living out of evaluating the impact of chemical exposures and other insults on the health of laboratory animals (we can't test humans after all). Rats and mice, members of the evolutionary order Rodentia, make up a large majority of these experimental animals. Ken Boschert, a veterinarian with Washington University's division of comparative medicine and the operator of a Web site called Net Vet (netvet.wustl.edu/) estimates that 99 percent of experimental animals nowadays are rats and mice, which are small, cheap to feed, and reproduce quickly. Rats and mice are also believed to share a closer evolutionary lineage to humans than other non-primate mammals. Yet, another familiar mammal, guinea pigs, are in many ways toxicologically and genetically more like humans than rats, mice and even our closest evolutionary cousins, the chimpanzee. This article will relate evidence from personal experiences and readings that suggest that guinea pigs are more likely to have shared a common designer than a common ancestor with humans.