To elucidate the mechanism of clomiphene action in the treatment of male infertility, we studied the effect of three-month clomiphene treatment (50 mg daily) on the radioimmunoassayable concentrations of gonadotropins (FSH, LH), prolactin (PRL), estradiol (E2), and testosterone (T) in seminal plasma and peripheral serum in eleven normogonadotropic subfertile men. Clomiphene therapy increased (p less than 0.01) the levels of FSH and LH in seminal plasma, but to a lesser extent (p less than 0.01) than in peripheral serum. PRL did not change is seminal plasma or peripheral serum. The seminal plasma and peripheral serum T concentrations doubled following clomiphene intake without any demonstrable correlation with the sperm count. The concentrations of E2 increased about ten times (p less than 0.005) in seminal plasma, a significantly greater (p less than 0.05) increase than that in the serum. There was a significant correlation (r = 0.804, p less than 0.01) between the sperm counts and seminal plasma E2 levels following clomiphene treatment. These results suggest that Sertoli cell activation may be a key event in clomiphene action in the treatment of male infertility.