Retinoic acid (RA) inhibits adipocyte differentiation of 3T3-L1 preadipocytes but is effective only early in adipogenesis. RA prevented induction of the adipogenic factors PPAR␥ and C/EBP␣. Using receptor-specific ligands, we determined that the effects of RA were mediated by liganded RA receptors (RARs) rather than retinoid X receptors. Preadipocytes expressed primarily RAR␣ and RAR␥; during adipocyte differentiation, RAR␣ gene expression was nearly constant, whereas RAR␥1 mRNA and protein levels dramatically decreased. Ectopic expression of RAR␥1 extended the period of effectiveness of RA by 24 to 48 h; RAR␣ expression had a similar effect, suggesting functional redundancy of RAR subtypes. Remarkably, RA inhibited differentiation when added after PPAR␥1 and PPAR␥2 proteins had already been expressed and resulted in the loss of PPAR␥ proteins from cells. By 72 to 96 h after the induction of differentiation, RA failed to prevent differentiation of even ectopic-RAR-expressing cells. Thus, the unresponsiveness of 3T3-L1 preadipocytes to RA after the induction of differentiation is initially due to the reduction in cellular RAR concentration rather than to the induction of PPAR␥. At later times cells continue along the differentiation pathway in a manner which is RA and RAR independent.