The effects of gestrinone (R 2323) on endometrial and endometriosis tissue concentrations of cytosol estrogen and progestin receptors and the activity of 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) were investigated in 11 patients operated on because of suspected external endometriosis. Serum concentrations of luteinizing hormone, follicle-stimulating hormone, estradiol (E2), progesterone, testosterone (T), and sex-hormone-binding globulin (SHBG) were also investigated. After one control cycle, the patients received 2.5 mg of oral gestrinone twice weekly from the fifth day of the first treatment cycle until the eighth day of the second treatment cycle, the day of operation being day 10. Treatment with gestrinone decreased serum concentrations of T during the second treatment cycle and effected a major decrease in SHBG during both treatment cycles, resulting in highly increased free T and free E2 indices. The effects of gestrinone on the endometrium, a decrease in estrogen and progestin receptors, and induction of 17 beta-HSD are characteristic progestin actions. These parameters remained unchanged in endometriosis tissue. Our data indicate that gestrinone has effects that are typical of androgens and progestins in patients with endometriosis.