The AMP-activated protein kinase (AMPK) activator, 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR), has been found to inhibit the differentiation of 3T3-L1 adipocytes, if added at an early phase of differentiation. AICAR blocks the expression of the late adipogenic markers, fatty acid synthase and acetyl-CoA carboxylase, and of the transcription factors, C/EBP alpha and PPAR gamma. It also inhibits early clonal expansion of pre-adipocytes, prevents the fall in C/EBP beta expression during the intermediate stage of differentiation and inhibits the late phase expression of CHOP-10, an antagonist of C/EBP beta. These data suggest a possible inhibitory role for AMPK in the process of adipose differentiation and suggest that AMPK might be a target to block adipogenesis.