Caveolin-1, the first member of caveolin family reported, is recognized as the structural component of caveola, a plasma membrane invagination or vesicles that are a subcompartment distinct from clathrin-coated pits. This protein is also known to be involved in cholesterol trafficking. The aim of this study was to determine the expression of caveolin-1 in adult rat Leydig cells. Testis sections incubated with an antibody to caveolin-1 showed, by immunohistochemistry, a moderate number of Leydig cells with different degrees of immunoreaction and a strong reaction in endothelial cells and in the lamina propia of seminiferous tubules. Caveolin- 1 was detected in the cell cytoplasm with a granular pattern and on the cell surface of Leydig cells cultured 24 h on uncoated, laminin-1 or type IV collagen coated coverslips. We also observed a milder reaction in 3 h cultures. Immunoreaction was also detected in Leydig cells with an antibody to tyrosine-phosphorylated caveolin-1. By double immunofluorescent technique, we observed co-localization of caveolin- I and 313-hydroxysteroid dehydrogenase. Western blot analysis revealed a band of about 22 kDa molecular weight that was recognized with both caveolin-1 and tyrosine-phosphocaveolin-1 antibodies. Caveolin-l is one of a few proteins with a demonstrated ability to bind cholesterol in vivo. In this context, the presence of caveolin- in Leydig cells may be related to cholesterol traffic--a rate limiting step in steroid biosynthesis.