The effects of cyproterone acetate (CA) on reproductive functions in normal human males were studied. 6 volunteers received 5 or 10 mg CA over a 20-week period. The treatment caused a gradual decrease in the number of spermatozoa and their motility, and an increase in the percentage of nonmotile, abnormal, and immature sperms. There was also a marked inhibition of sperm transport of motile sperm through cervical mucus, as determined by Kremer's test. Semen levels of acid phosphatase, sialic acid, and glycerylphosphorylcholine progressively decreased, though semen levels of fructose were not markedly altered. There were no marked changes in levels of SGOT, SGPT, serum alkaline phosphatase, blood urea, and hematocrit values. The possible mode of action of CA and its potential as a male contraceptive agent are discussed.