Discovery of the endogenous opiate system of the brain has revolutionized the study of pain and pain management. In this study a convenience sample of 10 pregnant women, 16 nonpregnant women, and 18 men were studied to determine if changes in plasma beta-endorphin-like immunoreactivity affected pain perception during labor. Pregnant women had plasma levels of beta-endorphin significantly higher than nonpregnant women at the midpoint of their menstrual cycle, t = 3.74, df = 31, p = .007. Self-reported pain perception scores were not correlated with plasma beta-endorphin levels. However, as labor progressed, the women reported increased discomfort between contractions and during contractions while beta-endorphin levels increased only slightly. Close examination of the pain pattern indicates that pain perceived by the women between contractions increased at a greater rate than during contractions. This pattern suggests that opiate-active beta-endorphin may increase the ability of women to tolerate acute pain.