In six rhesus monkeys, ovulation was induced by human menpausal gonadotropin and human chorionic gonadotropin and the oviducts were catheterized for continuous collection of oviductal fluid. Oviductal fluid and plasma samples were obtained daily from days 8 to 16 of therapy and were analyzed for estrone (E 1 ), 17β-estradiol (E 2 ), androstenedione (A), testosterone (T), and progesterone (P). Ovulation was confirmed by noting increases in plasma progesterone levels and by direct inspection of the corpus luteum (C-L) during laparotomy. Two monkeys ovulated with one or two C-L, two monkeys had “superovulation” with multiple C-L, and the other two monkeys failed to ovulate. The oviductal fluid volume was larger during the preovulatory phase than during the postovulatory phase of the cycle. The concentrations of steroid hormones in the oviductal fluid were lowerthan those in the plasma prior to ovulation, but E 1 and T rose to or above plasma levels during the postovulatory period. Following ovulation, there was a gradual increase in the oviductal fluid concentrations of E 1 , T, and P, whereas the concentrations of E 2 and A decreased. In anovulatory cycles, all steroid hormone levels fluctuated without trend throughout the observation period. In superovulated cycles, the concentrations of all steroids in the oviductal fluid increased rapidly following ovulation and reached levels that were 3 to 8 times those in the plasma. By using equilibrium dialysis, the nonprotein-bound (free) fraction of steroid in the oviductal fluid was determined. The mean (± standard error) values of percentage of free E 1 , E 2 , A, T, and P, respectively, were 62.3 ± 3.2, 64.4 ± 2.6, 73.8 ± 1.9, 64.1 ± 2.1, and 46.9 ± 2.7%. The corresponding data for percentage of free plasma steroid, respectively, were: 7.62 ± 0.30, 3.92 ± 0.14, 9.84 ± 0.49, 2.68 ± 0.10, and 3.61 ± 0.13%. Thus, high concentrations of biologically active free steroid hormones, found in significant amounts after ovulation, are present in the oviductal environment and might influence its function during this crucial period in reproduction. In addition, the qualitative difference between plasma and oviductal hormones suggests a mechanism other than passive diffusion for their presence in oviductal fluid.