The adipocyte-derived hormone leptin is a central modulator of food intake, metabolism and neuroendocrine functions. It is also involved in a physiological loop linking the activity of the hypothalamo-pituitary-adrenal axis and adipose tissue. At the adrenal level, leptin has been shown to antagonize the effects of ACTH on glucocorticoid biosynthesis by decreasing the expression of various enzymes of the steroid biosynthetic pathway. The steroidogenic acute regulatory protein regulates cholesterol delivery to the P450(scc) enzyme, a process that is rate limiting in steroid hormone biosynthesis. We have demonstrated here that leptin significantly inhibits the expression of steroidogenic acute regulatory protein in primary cultures of rat adrenocortical cells. This inhibition was observed at both the protein and mRNA levels. In contrast, leptin was not found to interfere with the expression of the cytosolic enzyme cholesterol ester hydrolase or with that of the mitochondrial enzyme P450(scc). In addition, we observed the anticipated stimulation of cAMP production by ACTH in the presence of leptin, suggesting that it does not interfere with intracellular ACTH signaling. In summary, our data provide evidence that the interplay existing between leptin and ACTH in vivo is mediated at least partially via a direct and opposite modulation of steroidogenic acute regulatory protein, a key factor in the adrenal steroid biosynthetic pathway. This effect of leptin could also be relevant to other steroidogenic tissues.