Adenovirus-induced hyperleptinemia causes rapid disappearance of body fat in normal rats, presumably by up-regulating fatty acid oxidation within white adipocytes. To determine the role of peroxisomal proliferation-activated receptor (PPAR)α expression, which was increased during the rapid loss of fat, we infused adenovirus–leptin into PPARα^−/− and PPARα^+/+ mice. Despite similar degrees of hyperleptinemia and reduction in food intake, epididymal fat pad weight declined 55% in wild-type but only 6% in PPARα^−/− mice; liver triacylglycerol fell 39% in the wild-type group but was unchanged in PPAR^−/− mice. Carnitine palmitoyl transferase-1 mRNA rose 52% in the wild-type mice but did not increase in PPARα^−/− mice. PPARγ coactivator-1α rose 3-fold in the fat and 46% in the liver of wild-type mice but was unchanged in PPARα^−/− mice. Although AMP-activated protein kinase could not be implicated in the lipopenic actions of hyperleptinemia, acetyl CoA carboxylase protein was reduced in the liver of wild-type but not in PPARα^−/− mice. Thus, in PPARα^−/− mice, up-regulation of carnitine palmitoyl transferase-1 mRNA in fat, down-regulation of acetyl CoA carboxylase in liver, and up-regulation of PPARγ coactivator-1α mRNA in both tissues are abolished, as is the reduction in their triacylglycerol content.