Silastic tube-type implants containing diethylstilbestrol (DES) were placed subcutaneously in five male white-tailed deer (Odocoileus virginianus). Implants were recovered from four deer and determined to have a mean daily release rate of 205 ug. At this rate DES drastically suppressed antler growth, but the velvet was shed and bone antlers were formed. Treated males had significantly lower testes weights than controls. However, DES affected spermatogenesis but the response was not uniform, ranging from almost complete cessation to limited sperm production. The round spermatid population was significantly (P < 0.05) reduced in the DES group. Morphologically, the Leydig cells in treated deer appeared different from those in the control animals. The epididymides were unaffected by treatment. The treatment affected the secretory cells of the Cowper's and prostrate glands in three of the four deer and the secretory cells of the seminal vesicles in all four. Because of abnormal antler development, research with DES as a male deer antifertility agent was discontinued.