We have isolated a cDNA corresponding to the hamster peroxisome proliferator-activated receptor haPPAR gamma, a member of the steroid nuclear hormone receptor superfamily of transcription factors. haPPAR gamma mRNA is highly expressed in adipose tissue, and is expressed in lung, heart, kidney, liver and spleen to a lower extent. Thus, haPPAR gamma may function in activating the transcription of target genes in a variety of tissues, including those not particularly subjected to peroxisomal beta-oxidation. haPPAR gamma binds efficiently in the presence of retinoid X receptor alpha (RXR alpha) to a peroxisome proliferator response element (PPRE) first identified in the acyl-CoA oxidase (ACO) promoter, the rate-limiting enzyme of peroxisomal beta-oxidation. The gene (ACO) encoding this enzyme has been previously shown to be under the transcriptional control of mouse PPAR (mPPAR). Although binding of haPPAR gamma/RXR alpha on the PPRE of the ACO promoter in vitro is similar to that observed for mPPAR/RXR alpha, we show that the transcriptional activities of mPPAR and haPPAR gamma are regulated differently in vivo in response to peroxisome proliferators and heterodimerization with RXR.