We describe the molecular cloning and expression of cDNAs encoding human PPARγ1 and PPARγ2. Our sequences are distinct from the published sequence at 3 positions, resulting in nonconservative amino acid substitutions. In humans, PPARγ mRNA is expressed in spleen, bone marrow, liver, testis, skeletal muscle and brain, in addition to fat. Three thiazolidinediones were found to 1) displace a radiolabeled thiazolidinedione from both receptors with essentially the same IC₅₀s and 2) to transactivate both PPARγ isoforms with similar EC₅₀s in transient cotransfection assays utilizing the adipocyte-specific aP2 promoter. Saturating concentrations of these 3 thiazolidinediones altered the conformation ofin vitrosynthesized PPARγ protein producing a 27 kDa protease-resistant fragment. These results indicate that the antidiabetic effects of thiazolidinediones in humans are likely to be mediated via binding to and transactivation of PPARγ1 and γ2.