Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily implicated in adipocyte differentiation. The observations that PPARα is a regulator of hepatic lipid metabolism and that the insulin-sensitizing thiazolidinediones are ligands for PPARγ suggest that cross-talk might exist between insulin signaling and PPAR activity, possibly through insulin-induced PPAR phosphorylation. Immunoprecipitation of endogenous PPARα from primary rat adipocytes prelabeled with [32P]-orthophosphate and pretreated for 2 h with vanadate and okadaic acid demonstrated for the first time that PPARα is a phosphoprotein in vivo. Treatment with insulin induced a time-dependent increase in PPAR phosphorylation showing a 3-fold increase after 30 min. Insulin also increased the phosphorylation of human PPARα expressed CV-1 cells. These changes in phosphorylation were paralleled by enhanced transcriptional activity of pPARα and γ. Transfection studies in CV-1 cells and HepG2 cells revealed a nearly 2-fold increase of PPAR activity in the presence of insulin. In contrast, insulin had no effect on the transcriptional activity of transfected thyroid hormone receptor in CV-1 cells, suggesting a PPAR-specific effect. Thus, insulin stimulates PPARα phosphorylation and enhances the transcriptional activity of PPAR, suggesting that the transcriptional activity of this nuclear hormone receptor might be modulated by insulin-mediated phosphorylation.