Effects of zeranol on scrotal circumference, serving ability, semen characteristics, and postmortem measurements of the genital organs were determined in beef bulls from 9 to 20 months of age. Group 1 (n = 5) served as a nonimplanted control group. Group 2A (n = 5) was implanted with 36 mg of zeranol at birth and at 3 and 6 months of age. Group 2B (n = 5) was implanted with 36 mg of zeranol every 3 months from birth through 18 months of age. Scrotal circumference was adversely affected by zeranol in groups 2A and 2B, but values approached those of group 1 with increasing age. Serving ability was also affected adversely but tended to recover with increasing age. Semen quality was low in groups 2A and 2B and did not improve with increasing age. There was no difference in testicular weight, vesicular gland weight, and penis length among groups when bulls were slaughtered at 20 months of age. Epididymal weight was greater in group-2B bulls and was most likely a consequence of epididymal lesions. Histologic examination of the genital organs revealed that zeranol induced adenomyosis and sperm granulomas in the caudae epididymidis and markedly altered the structure of the sexual accessory glands of bulls in groups 2A and 2B. Alterations in the vesicular glands were characterized by reduced alveolar development and an increase in connective tissue. Low epithelium associated with focal areas of squamous metaplasia were common in the prostate of groups 2A and 2B bulls. Lesions in the bulbourethral glands were characterized by low glandular epithelium, focal areas of squamous metaplasia, cystic collecting ducts, and an increase in connective tissue. Groups 2A and 2B had more abnormal seminiferous tubules than did group 1. Lesions in groups 2A and 2B may have been direct effects of zeranol or may have resulted from reduced testosterone secretion.