Adipocyte differentiation is driven by the expression and activation of three transcription factor families: the differentially expressed CAAT/enhancer binding proteins (C/EBPs) alpha, beta, and delta; the helix-loop-helix adipocyte differentiation and determination factor-1; and peroxisome proliferator activated receptor gamma (PPARgamma), expressed as two isoforms, PPARgamma1 and the adipocyte-specific PPARgamma2. Overexpression of PPARgamma can induce adipocyte differentiation; therefore, we analyzed the expression of the two PPARgamma isoforms during early stages of differentiation to determine whether one was preferentially induced as an early determining event. Surprisingly, in the first 24 h, a 3-6-fold increase of PPARgamma2 mRNA was observed, whereas PPARgamma1 mRNA remained unchanged. PPARgamma1 was induced 1 day later. Overexpression of C/EBPbeta has also been shown to induce adipocyte differentiation. A C/EBP site was identified only in the human PPARgamma2 promoter. Its deletion blunted the response of PPARgamma2 promoter to cotransfected C/EBPbeta or methylisobutylxanthine treatment. We hypothesize that PPARgamma2 initiates adipocyte differentiation.