Peroxisome proliferators are a diverse group of chemicals that include several therapeutically used drugs (e.g., hypolipidemic agents), plasticizers and organic solvents used in the chemical industry, herbicides, and naturally occurring hormones. As the name implies, peroxisome proliferators cause an increase in the number and size of peroxisomes in the liver, kidney, and heart tissue of susceptible species, such as rats and mice. Long-term administration of peroxisome proliferators can cause liver cancer in these animals, a response that has been the central issue of research on peroxisome proliferators for many years. Peroxisome proliferators are representative of the class of nongenotoxic carcinogens that cause cancer through mechanisms that do not involve direct DNA damage. The fact that humans are frequently exposed to these agents makes them of particular concern to government regulatory agencies responsible for assuring human safety. Whether frequent exposure to peroxisome proliferators represents a hazard to humans is unknown; however, increased cancer risk has not been shown to be associated with long-term therapeutic administration of the hypolipidemic drugs gemfibrozil, fenofibrate, and clofibrate. To make sound judgments regarding the safety of peroxisome proliferators, the validity of extrapolating results from rodent bioassays to humans must be based on the agents' mechanism of action and species differences in biologic activity and carcinogenicity. The peroxisome proliferatoractivated receptor a (PPARa), a member of the nuclear receptor superfamily, has been found to mediate the activity of peroxisome proliferators in mice. Gene-knockout mice lacking PPARa are refractory to peroxisome proliferation and peroxisome proliferator-induced changes in gene expression. Furthermore, PPARa-null mice are resistant to hepatocarcinogenesis when fed a diet containing a potent nongenotoxic carcinogen WY-14,643. Recent studies have revealed that humans have considerably lower levels of PPARa in liver than rodents, and this difference may, in part, explain the species differences in the carcinogenic response to peroxisome proliferators.