The study was done to assess the clinical performance and in vivo steroid release rate of 3-keto-desogestrel subdermal implants designed to deliver 5 different doses of the progestin. Volunteers were healthy women of proven fertility who provided blood samples at scheduled intervals during treatment. No pregnancy occurred in 514 woman-months in users of implants delivering 30 and 40 micrograms per day of 3-keto-desogestrel. Three pregnancies, one ectopic, were observed in 109 woman-months recorded with implants delivering 20 micrograms per day or less. Ovulation was inhibited, as judged by depressed progesterone levels, in 57 of 59 (97%) blood samplings in women whose 3-keto-desogestrel plasma levels were greater than 0.28 nmol/L and in 39 of 75 (52%) of cases with lower levels. Users of 4 cm implants manufactured by The Population Council, New York, showed mean levels above 0.28 nmol/L until 18 months of use. Levels achieved with 4.4 cm implants manufactured by Organon, Oss, Holland, were less consistent. No changes were observed in the plasma lipoprotein pattern or clinical chemistry during treatment. The main complaint was the occurrence of bleeding irregularities, particularly with the lower doses. Ovarian cysts found during pelvic examination in 11 (22%) subjects disappeared spontaneously within 7-90 days. 3-keto-desogestrel implants releasing around 40 ug/day and providing plasma levels around 0.28 nmol/L afford efficient contraceptive protection.