Beta-endorphins are endogenous opioid substances produced by the pituitary gland and placenta. The aims of this project were to longitudinally follow plasma levels of beta-endorphin during pregnancy in women with a healthy pregnancy and to investigate whether plasma levels of beta-endorphin in late pregnancy are associated with need for additional pain medication beyond nitrous oxide during labor. Plasma samples from 45 women were collected at gestational weeks 10, 25, 28, 33 and 37, and beta-endorphin was analyzed by radioimmunoassay (RIA). Plasma levels of beta-endorphin displayed a significant decrease in gestational weeks 28 and 33 compared to week 10, followed by a subsequent increase between gestational weeks 28 and 37. However, there was no change in levels of beta-endorphin between gestational weeks 10 and 37. Low levels of beta-endorphin at the end of pregnancy were associated with need for additional pain medication beyond nitrous oxide during labor, although the causal relationship is unclear.