A group of receptors termed peroxisome proliferator activated receptors (PPAR), belonging to the nuclear hormone receptor supergene family, might be crucial in explaining how a diverse group of apparently unrelated chemicals induce peroxisomal proliferation and a change in the expression of several genes. The activation of these PPAR by peroxisome proliferators, as well as by fatty acids, might reconcile the apparent discrepancy between the two prevailing theories that explain peroxisome proliferation, i.e. the receptor and the fatty acid theory. Although the exact physiological role of PPAR is not yet known, these receptors might have a far more general function than strictly regulating peroxisomal gene expression by changing the expression of numerous genes in response to developmental and nutritional challenges. Much work, however, remains to be performed before a complete picture will emerge.