Peritoneal fluid was collected at laparoscopy in women during the menstrual cycle and was assayed for protein and steroid hormone content. The total protein concentration in peritoneal fluid and the concentrations of the steroid hormone-binding proteins, transcortin and sex hormone-binding globulin, the polypeptide hormones, LH, FSH, and PRL, correlated with the plasma concentration but were lower; they were, respectively, 68%, 71%, 68%, 42%, and 34% of the plasma concentration. The concentrations of steroid hormones secreted by the ovary, i.e. 17 beta-estradiol, progesterone, androstenedione, and testosterone, were always equal or higher in peritoneal fluid than in plasma. In contrast, the concentrations of cortisol, a nonovarian steroid hormone, was 40% lower in peritoneal fluid than in plasma. No cyclic variations were observed in the peritoneal fluid concentrations of androstenedione and testosterone, two steroid hormones secreted by the stromal component of the ovary. On the contrary, the concentrations of 17 beta-estradiol and progesterone secreted by the follicular apparatus of the ovary increased sharply in peritoneal fluid after ovulation, reaching values of 44000 pg/ml and 3000 ng/ml, respectively. They declined progressively, whereas in plasma, peak concentrations were achieved only in the midluteal phase. In conclusion, the concentrations of 17 beta-estradiol and progesterone are much higher in peritoneal fluid than in plasma for at least 1 week after ovulation. We suggest that the secretion of the early, not yet vascularized, corpus luteum is directed preferentially toward the peritoneal cavity, creating a specific hormonal environment for the released oocyte and the oviduct.