Ingestion of rosemary (Rosmarinus officinalis L.) by two groups of adult Sprague-Dawley rats at levels of 250 and 500 mg/kg body wt for 63 days was investigated for its effects on fertility. Body weight and absolute and relative testes weights were not affected, but the average weights of epididymides, ventral prostates, seminal vesicles, and preputial glands decreased significantly. A significant decline in spermatogenesis in testes due to a decrease in the number of primary and secondary spermatocytes and spermatids in treatment group 2 (500 mg/kg) is attributed to a significant decrease in testosterone. Sperm motility and density were also significantly decreased in the cauda epididymis and in the testes of rosemary-treated male rats in group 2. In addition, the treatment markedly increased the number of fetal resorptions in female rats impregnated by group 2 males, thereby reducing their fertility.