The effect of Anandron, a nonsteroidal antiandrogen, on the reproductive system and fertility of adult male rats was studied. Administered at a daily oral dose of 5 mg and 10 mg (per rat) for 30 days, it caused a significant increase in the plasma testosterone levels. Spermatogenic arrest in about 20 to 50% of the tubules in 9 out of 16 rats and stimulation of Leydig cells was observed in rats treated with the higher dose. Although a reduction occurred in accessory sex organ (epididymis, seminal vesicles, SV; ventral prostate, VP; dorsal prostate, DP and coagulating gland, CG) weight, no parallel reduction was evident in the secretory indices of the epididymis (glycerylphosphorylcholine and sialic acid), VP (alkaline phosphatase) and CG (fructose). However, there was a reduction in the total content per organ of these constituents. Females mated with treated males showed postimplantation loss indicating an adverse effect of Anandron during spermiogenesis. The results suggest that the peripheral antiandrogenic potency of Anandron in intact animals is insufficient to completely neutralize the elevated levels of androgens. A microdose (1 microgram) of estradiol efficiently neutralized the central stimulatory effect of Anadron and potentiated its antiandrogenic action. The potential use of such a combination for 'Fertility Regulation' in male is discussed.