This study was performed to document for the first time peritoneal fluid volumes and steroid hormone levels in baboons with and without endometriosis. A laparoscopy was carried out in 19 female baboons (11 with a normal pelvis and 8 with histologically proven spontaneous minimal/mild endometriosis) during 64 cycles in the early luteal phase. Peritoneal fluid was measured and aspirated. The pelvis was examined for evidence of recent ovulation and endometriosis. Peritoneal fluid and serum were assayed for 17 beta-estradiol and progesterone. The mean volume of peritoneal fluid and its concentration of 17 beta-estradiol and progesterone were 2.6 ml, 679 pmol/l and 64 nmol/l, respectively. No differences were observed between animals with or without either endometriosis or luteinized unruptured follicle syndrome. The peritoneal fluid and serum steroid levels were comparable to those found in women. The results presented are similar to those obtained in women with endometriosis and this suggests that the baboon can be used for the study of this disease.