PIP: A group of 12 women received subdermal implants of d-norgestrel (average dose per capsule, 33.9+ or -.7 mg; 203.4 mg/set of 6). On Day 5 of menstrual cycle, 6 capsules (dimethyl polysiloxane) were implanted subcutaneously and radially from a fixed point through a small incision in the forearm. Each woman kept daily records of bleeding or spotting patterns. No evidence of local inflammation was found at the insertion site. After implant insertion, mean menstrual cycle intervals varied markedly, ranging from 17-64 days. Some individuals had marked cycle variations in cycle intervals, indicating the unpredictability of the bleeding. 9/12 increased the number of bleeding days, ranging from 14-68 more days of bleeding than normally expected. The calculated in vivo release rates per woman ranged from 23.4-151.2 mcg/day. Variability in the measured amount of d-norgestrel remaining in the capsules after removal was markedly greater than the variability in measured amount before insertion (P .01). The variable release rates could not be related to bleeding patterns. After the 1st month of treatment (total, 1 year), serum levels of d-norgestrel fell from .65-.19 ng/ml at 1 year (P .001). Estradiol peaks occurred in several cycles in about half of the subjects though no pregnancies occurred. After capsule removal, return of ovulatory function took place within 4 weeks.