Gossypol treatment of endometriosis and uterine myomas in 15 cases was started with a daily dose of 20 mg of gossypol-acetic acid twice a day for 20 days as a loading dose, followed by 40 mg (20 mg twice weekly). A maintenance dose of 20 mg was given weekly. The total course of treatment lasted about 6 months. Light and electron microscopic examination of the endometrium was carried out before treatment and 2, 3, 4, and 6 months after gossypol treatment. After gossypol treatment three different characteristics of the endometrium were noted: (1) irregular secretory activity, (2) proliferative activity, and (3) atrophy. On electron microscopic examination it was shown that there were important changes of the organelles, indicating suppressed secretory activity. Based on the above results, the nature and significance of the ultrastructural changes of the endometrium induced by gossypol were analyzed and the possibility of developing a new field of research in female contraception with gossypol was considered.