Abstract: The cotton seed meal was shown to possess marked anti-fertility effect in male rats. Its active principle was found to be gossypol. The minimal effective anti-fertility doses of gossypol for male rats were 10～12 mg/kg/day for 4～5 weeks when administered orally. The fertility of the animals recovered 4～5 weeks after cessation of the medication. Gossypol neither altered the mating behaviour and the function of accessory glands in rats, nor changed the plasma testosterone levels in rats and rhesus monkeys. It did not affect the growth rate and the fertility of the offsprings. After long-term administration of gossypol to rats, no obvious abnormalities were found in the electrocardiogram. In some of these treated rats, the levels of serum glutamicpyruvic transaminase and gammaglobulin were raised slightly but restored to normal after cessation of the medication. The histological examinations of the liver tissues of a few treated rats showed that there were pathological changes in the hepatic cells. Since d-gossypol did not possess any significant anti-fertility effect or toxicity, it was suggested that these actions of gossypol were related to its levo-rotatory substance only. When the-CHO and-OH groups in the structure of gossypol were replaced by other groups, the anti-fertility effect was lost.