Substance P is a neuropeptide that has been identified in the ovary, fallopian tube, uterus, and vagina and in the hypothalamic-pituitary axis in both an animal model and human ovaries. We sought to determine if substance P is present in peritoneal fluid and, if so, whether it correlated with the cause of infertility. Its presence was determined by radioimmunoassay in the peritoneal fluid of 66 patients undergoing diagnostic laparoscopy for clinical indications related to infertility. Total volume of peritoneal fluid and cycle day were recorded; patients were evaluated in groups according to diagnosis: endometriosis (n = 24), pelvic adhesions (n = 18), and normal controls (n = 24). The level of substance P (mean +/- SEM) was 122 +/- 19 pg/ml for endometriosis and 130 +/- 19 pg/ml for pelvic adhesions. These values were not significantly different from the normal controls (130 +/- 25 pg/ml). There was no significant difference in levels between follicular and luteal phase of the menstrual cycle. We conclude that substance P is present normally in peritoneal fluid and that its levels are not affected by pelvic endometriosis or adhesions.