Healthy adipose tissue function depends on adipogenesis. The capacity to form new adipocytes prevents the emergence of insulin-resistant hypertrophied adipocytes, as well as the deleterious lipid deposition in muscle, liver, and pancreas. It is therefore important to understand how adipogenesis is modulated. Platelet-derived growth factor (PDGF) is anti-adipogenic, but the stage of differentiation that it targets, and the signaling pathways that it triggers, are not defined. We have studied the inhibitory effect of PDGF on murine 3T3-L1 preadipocyte and human preadipocyte differentiation. There was a significant attenuation in the protein expression of the adipogenic transcription factors, PPARγ and C/EBPα, as well as in the levels of later differentiation markers, including adiponectin, aP2, and fatty acid synthase. PDGF treatment resulted in the persistence of PDGF receptor and PKCα expression, in contrast to the expected downregulation of both proteins that occurs during differentiation. Inactivation of conventional PKC isoforms, by bisindolylmaleimide I or PKC pseudosubstrate M20–28, partially reversed the inhibition of 3T3-L1 and human preadipocyte differentiation by PDGF, as assessed by fatty acid synthase expression and morphological appearance. © 2005 Wiley-Liss, Inc.