Serum and peritoneal fluid inhibin levels were measured by radioimmunoassay throughout the menstrual cycle in 14 women with endometriosis and in 16 controls. In controls, serum values (+/- standard error of the mean) increased from the early follicular phase (49.8 +/- 6.5 microLEq/mL) to the late follicular phase (178 +/- 37.8 microLEq/mL) and the luteal phase (346 +/- 98.3 microLEq/mL). Peritoneal fluid inhibin levels were several-fold higher than those in serum and reached a maximum value during the late follicular phase (early follicular: 2404 +/- 85 microLEq/mL, late follicular: 22,922 +/- 7145 microLEq/mL, luteal: 5195 +/- 1959 microLEq/mL). There was no difference in peritoneal fluid or serum inhibin concentrations between patients with and without endometriosis. These findings suggest that human gametes in the fallopian tube may be exposed to a very high concentration of inhibin. The lack of difference in inhibin concentrations between patients with and without endometriosis suggests that this hormone does not play a role in endometriosis-related infertility.