Three bis(dichloroacetyl) diamine compounds (Win 13,099, Win 17,416, Win, 18,446) have been studied for their effects on testicular physiology in human volunteers. These studies show that: 1. 1. Win 13,099, Win 17,416, and Win 18,446 do not inhibit the human pituitary and do not suppress Leydig-cell activity. 2. 2. Each of these compounds suppresses sperm output and motility and alters sperm morphology, as demonstrated by seminal fluid analyses. Histologic study of testicular biopsy specimens reveals suppression of spermatogenesis. The suppressive action of Win 17,416 is, perhaps, incomplete. Win 13,099 and Win 18,446 each reduce the sperm counts to below 4 million per cubic centimeter within 8–11 weeks. There appears to be little other manifestation of metabolic activity. 3. 3. Depression of spermatogenesis appears to be reversible after discontinuance of the drugs. For example, complete recovery occurred after administration of Win 13,099 for as long as 27 weeks. 4. 4. Although the exact mode of action is not known, it is postulated that the effects produced result from a direct action on the process of germinal-cell maturation, at the site of the germinal epithelium, and without concurrent pituitary suppression.