Coregulators have been suggested to act as a bridging apparatus between nuclear receptors and the transcriptional machinery. The orphan receptor SF-1 plays a role in controlling the basal and cAMP-stimulated expression of the human steroidogenic acute regulatory protein gene. DAX-1 is the gene responsible for X-linked adrenal hypoplasia congenita and blocks steroid biosynthesis by impairing the expression of steroidogenic acute regulatory protein. In the present study we examined the role of coregulators in the actions of SF-1 and DAX-1 on the human steroidogenic acute regulatory protein promoter. We found that the coregulator RIP 140 interacts with SF-1 in the yeast two-hybrid system. Glutathione-S-transferase pull-down assays and coimmunoprecipitations confirmed the interaction between RIP 140 and SF-1. RIP 140 was also shown to interact with DAX-1. When an RIP 140 expression vector was introduced into Y-1 cells, basal and cAMP-stimulated human steroidogenic acute regulatory protein promoter activities decreased. The inhibitory effect of RIP 140 on human steroidogenic acute regulatory protein promoter activity was dependent upon the presence of SF-1. The cAMP response of an SF-1 response element was inhibited by both RIP 140 and DAX-1 expression vectors at low concentrations of plasmids. We conclude that RIP 140 binds to the orphan nuclear receptor SF-1 and DAX-1 and modulates their actions on the human steroidogenic acute regulatory protein promoter.