In addition to their role in controlling water and salt homeostasis, recent work suggests that aldosterone and mineralocorticoid receptors (MR) may be involved in adipocyte biology. This is of particular relevance given the role of MR as a high-affinity receptor for both mineralocorticoids and glucocorticoids. We have thus examined the effect of aldosterone and MR on white adipose cell differentiation. When cells are cultured in a steroid-free medium, aldosterone promotes acquisition of the adipose phenotype of 3T3-L1 and 3T3-F442A cells in a time-, dose-, and MR-dependent manner. In contrast, late and long-term exposure to dexamethasone inhibits adipocyte terminal maturation. The aldosterone effect on adipose maturation was accompanied by induction of PPARgamma mRNA expression, which was blocked by the MR antagonist spironolactone. Under permissive culture conditions, specific MR down-regulation by siRNAs markedly inhibited 3T3-L1 differentiation by interfering with the transcriptional control of adipogenesis, an effect not mimicked by specific inactivation of the glucocorticoid receptor. These results demonstrate that MR represents an important proadipogenic transcription factor that may mediate both aldosterone and glucocorticoid effects on adipose tissue development. MR thus may be of pathophysiological relevance to the development of obesity and the metabolic syndrome.