A total of 134 women were studied, 75 of whom were using cyclic steroid contraception (ethinyl estradiol, 0.05 mg plus norgestrel, 0.5 mg) and 59 of whom had normal ovulatory cycles. By radioimmunoassay techniques simultaneous determinations were made of total estrogens (TE) and progesterone (P) in both plasma and endometrium and of FSH and LH in plasma, covering all days of the pseudomenstrual and true menstrual cycles. As expected, the ovulating women showed a midcycle peak of plasma TE and an increase in plasma P thereafter, while no cyclic variations were observed in the treated group. Correlations of endometrial concentrations of TE and P with plasma FSH and LH levels are discussed. In 13 of the 75 treated women elevations of endometrial P concentrations were present, presumably demonstrating the “escape phenomenon.” The possible future value of endometrial hormone determinations is suggested.