A total of 2335 women have accumulated 58,135 cycles of exposure to Implanon--a single-rod contraceptive implant containing 68 mg of etonogestrel with a 3-year duration of action. To assess this agent's pharmacokinetics, 8 women received 3 bolus injections: the first 1-2 months before implant insertion, the second after 12 months of use with the implant still in place, and the third after implant removal at 24 months. Etonogestrel concentrations decreased gradually over the 2 years of use. The absorption rate of about 30 mcg/day after 3 months of use decreased to 30 mcg/day at the end of 2 years. With a bioavailability that remained constant at about 95% and a clearance of about 7.5 L/hour, it can be assumed that there is no accumulation of steroid drug and that decreased serum concentrations are caused only by a slight lowering in release rate over time. The half-life of elimination was 25 hours--significantly lower than the 41.7 hours associated with Norplant implants.