Two-hundred-and-eighty Indonesian women were provided with Norplant2®, a levonorgestrel-releasing implant. At various time intervals, up to 5 years after Norplant® insertion, levonorgestrel and sex hormone binding globulin (SHBG) were assayed in blood plasma. After an initial burst of approximately 7 nmol/1, the levels of levonorgestrel rapidly decreased during the first month. The decrease continued to a nadir (1.1 nmol/1) which was reached 10 months later. The decrease was followed by an increase to a broad peak of 1.5 nmol/1 which was reached 2 years after insertion. Thereafter, a slow-decrease at a rate of approximately 18 pmol/month was seen. SHBG levels decreased significantly already 1 week after insertion. A nadir of levels (25 nmol/1) was reached 3 months later. The levels increased slowly again and remained constant (32 nmol/1) from about 15 months to 5 years. During the entire period of study highly significant correlations of levonorgestrel with SHBG were seen. In another group of 25 women the levels of levonorgestrel and SHBG were studied before and one week after insertion of Norplant.® A significant correlation (r = 0.77) was found between the preinsertion levels of SHBG and postinsertion levels of levonorgestrel.2Norplant® is a trademark registered by the Population Council, New York, USA.