Serum estrone, estradiol, progesterone, FSH, and LH levels were studied after the implantation of estradiol pellets in female volunteers in the reproductive age group. The estradiol pellets were implanted at six-month intervals. The first implant consisted of four pellets (25 mg each). Subsequent implants consisted of a reduction by one pellet as compared to the previous implant. The mean serum estradiol levels during the six-month period of implantation or at the end of six-month intervals were in the premenopausal range and did not differ significantly during the study. The serum estrone levels showed a tendency to increase with time even though the number of pellets implanted was decreasing, apparently a result of incomplete absorption of the implanted estradiol during the six-month period. The results of serum progesterone determinations and basal body temperature records indicate that the suppression of ovulation takes place most consistently during the second and subsequent months after estradiol pellet implantation.