The purpose of this study was to evaluate the effect of daily administration of an anabolic steroid, metandienone, on spermatogenesis. Fifteen well-trained male athletes were prescribed orally 15 mg of metandienone for two months. Semen specimens before, after one month and after two months use of metandienone were studied. The sperm density per ml decreased 46% and 73%, respectively, the latter value being highly pathologic (25 ± 22 × 10 ° /ml). Three of the subjects became azoospermic and one of them had only 1 million sperms/ml after two months use of the drug. The percentage of motile cells decreased in two months about 30%. The percentage of sperms with normal configuration decreased significantly both in one and two months, from 73 ± 8% to 65 ± 5% and 42 ± 23%, respectively. The percentage of sperms with amorphous head increased about 100% during two months use of metandienone. Other kinds of anomalies in the sperms also increased significantly. The “fertility index” increased in two months to a value which is highly pathological. Acid phosphatase activity in semen decreased significantly during two months use, while fructose concentration changed significantly only after one month use of metandienone, but not after two months use. Metandienone is a sperm-suppressive agent, but it also has other effects on the quality of the semen. The changes were, however, reversible.