Following the oral administration of acetone extract of Foeniculum vulgare (fennel) seeds for 15 days is male rats, total protein concentration was found to be significantly decreased in testes and vas deferens and increased in seminal vesicles and prostate gland. There was a decrease in activities of acid and alkaline phosphatase in all these regions, except that alkaline phosphatase was unchanged in vasa. In female rats, oral administration of the extract for 10 days led to vaginal cornification and oestrus cycle. While moderate doses caused increase in weight of mammary glands, higher doses increased the weight of oviduct, endometrium, myometrium, cervix and vagina also. The results confirm the oestrogenic activity of the seed extract.