Immunoreactive (IR)-beta-endorphin, ACTH and cortisol from the plasma of 88 pregnant women during gestation, and 28 women with spontaneous labor were measured. The concentrations of ACTH, IR-beta-endorphin and cortisol gradually increased during gestation. There were significant correlation between ACTH and IR-beta-endorphin (r = 0.79, p less than 0.0001), as well as between ACTH and cortisol (r = 0.54, p less than 0.0001) during gestation. Maternal ACTH and IR-beta-endorphin concentrations rapidly increased during the course of labor, peaked immediately after delivery at 671.3 +/- 136.1 and 513.9 +/- 85.5 pg/ml, respectively, and then decreased two hours later. There was a highly significant correlation (r = 0.89, p less than 0.0001) between ACTH and IR-beta-endorphin during labor. The levels of cortisol were elevated during labor and showed no difference during labor, or two hours later. However, there was no significant relationship between ACTH and cortisol (r = 0.24) during labor. Chromatographic analysis was performed on the pooled plasma extracts from pregnant women at the various stage of labor. In ACTH analysis, a single peak was seen at the position of native ACTH in each case. All detectable immunoreactivities eluted in two peaks at the position of beta-LPH and beta-endorphin. However, beta-LPH to beta-endorphin ratio was almost the same in each case. These results suggest that 1) ACTH and IR-beta-endorphin are secreted concomitantly, probably by the maternal pituitary gland during gestation as well as in response to the stress of labor; 2) the lack of correlation between ACTH and cortisol is due to the difference in metabolic clearance rate of these hormones during labor; 3) neither peripheral nor intrapituitary conversion of beta-LPH to beta-endorphin is seen during gestation and labor.