T he scavenger receptor, class B, type I (SR-BI) binds HDL and mediates the selective transfer of cholesteryl esters from HDL to cultured cells. The tissue distribution of SR-BI in mice suggests that this receptor may deliver HDL-cholesterol to the liver and to nonplacental steroidogenic tissues. To examine the role of SR-BI in vivo, we determined its tissue and cell type-specific expression pattern and regulation in rats. High levels of immunodetectable SR-BI were present in the adrenal gland, ovary, and liver. In pregnant animals, the mammary gland also expressed high levels of the protein. SR-BI was localized by immunofluorescence to the surfaces of steroidogenic cells in the zona fasciculata and zona reticularis of the adrenal gland and to the corpus luteal cells of the ovary. High-dose estrogen treatment dramatically reduced SR-BI in the liver and increased SR-BI in the adrenal gland and corpus luteal cells of the ovary. These estrogen-induced increases in SR-BI in the adrenal gland and ovary were accompanied by enhanced in vivo uptake of fluorescent lipid from HDL. The administration of human chorionic gonadotropin induced a dramatic increase in SR-BI in the steroidogenic Leydig cells of the testes. These findings suggest that SR-BI mediates physiologically relevant uptake of cholesterol from HDL to nonplacental steroidogenic tissues in vivo.