Ingestion of propranolol at 10 and 15 mg kg(-1) body weight for 35 days by adult male mice was investigated for its effects on fertility. Body weight and absolute and relative testes weights were reduced and the average weights of epididymis, ventral prostate and seminal vesicle decreased significantly. A significant decline of spermatogenesis in testes due to a decrease in the number of primary, secondary spermatocytes and spermatids in the treatment group 2 (15 mg kg(-1)) is attributed to a significant decrease in testosterone, LH and FSH. Sperm motility and density were also significantly decreased in the cauda epididymis and in the testes of group 2 treated male mice. In addition, the treatment markedly increased the number of fetal resorptions in female mice impregnated by the group 2 males, thereby reducing their fertility.