The ability of DNA sequence-specific transcription factors to synergistically activate transcription is a common property of genes transcribed by RNA polymerase II. The present work characterizes a unique form of intermolecular transcriptional synergy between two members of the nuclear hormone receptor superfamily. Heterodimers formed between peroxisome proliferator-activated receptor γ (PPARγ), an adipocyte-enriched member of the superfamily required for adipogenesis, and retinoid X receptors (RXRs) can activate transcription in response to ligands specific for either subunit of the dimer. Simultaneous treatment with ligands specific for both PPARγ and RXR has a synergistic effect on the transactivation of reporter genes and on adipocyte differentiation in cultured cells. Mutation of the PPARγ hormone-dependent activation domain (named τc or AF-2) inhibits the ability of RXR-PPARγ heterodimers to respond to ligands specific for either subunit. In contrast, the ability of RXR- and PPARγ-specific ligands to synergize does not require the hormone-dependent activation domain of RXR. The results of in vitro and in vivo experiments indicate that binding of ligands to RXR alters the conformation of the dimerization partner, PPARγ, and modulates the activity of the heterodimer in a manner independent of the RXR hormone-dependent activation domain.