During laparoscopy peritoneal fluid samples were collected for FSH, LH, PRL, 17 beta-estradiol, progesterone, and total protein determinations in 100 women with a normal menstrual cycle. The samples were collected between cycle day -6 and cycle day +9, with the serum LH peak as a point of reference (day 0). The period investigated was divided into seven phases. FSH and LH concentrations in the peritoneal fluid varied in a cycle-dependent pattern that reflected the pattern in serum. In every phase of the cycle, however, peritoneal fluid FSH and LH concentrations were higher than or equal to the serum levels. This finding contrasts with the physiological behavior of other proteins in the peritoneal fluid. Peritoneal fluid 17 beta-estradiol and progesterone levels also varied in a cyclic pattern, with an increase in concentration immediately after ovulation and a decrease after the midluteal phase. With the exception of 17 beta-estradiol levels during the preovulatory phase of the cycle, peritoneal fluid levels of 17 beta-estradiol and progesterone were always equal to or higher than serum levels. The increase in 17 beta-estradiol concentration in the postovulatory phase was more gradual than that in the progesterone concentration. The elevated peritoneal fluid levels of gonadotropins in the preovulatory phase of the menstrual cycle were the most striking finding of the present study. This together with the finding of high peritoneal fluid to serum ratios of steroid hormones after ovulation shed new light upon the surroundings in which follicular development, ovulation, and fertilization take place.