Clomiphene citrate was administered as a 50 mg oral daily dose to 44 normogonadotrophic (serum FSH 2-10 mIU/ml) subfertile men for 3 months. The treatment resulted in significant increases in FSH and LH concentrations, whereas prolactin remained unchanged. Serum testosterone and oestradiol both increased highly significantly. The increased testosterone levels suggest that the elevated LH levels had not led to "down regulation" of Leydig cell LH/hCG receptors, neither had the greatly increased estradiol led to depletion of these receptors. This is suggested to be a result of the blocking of testicular oestradiol receptors by the estrogen antagonist, clomiphene. Sperm count increased highly unchanged. The spermatic fluid concentrations of zinc and magnesium ions were also increased, whereas fructose remained unchanged. The katalytic activity of acid phosphatase in spermatic fluid increased highly significantly, whereas the concentration of the main prostate-specific acid phosphatase, as measured by a specific radioimmunological method, remained unchanged. Therefore, the increased Zn and Mg ion concentrations may be responsible for activation of acid phosphatase (s) in semen, or the treatment led to increased secretion of other prostatic acid phosphatase(s) than the main enzyme. However, it is clear that the secretion of the main prostatic acid phosphatase into semen is under different control than that of Zn++ and Mg++.