Abstract Several approaches have been used by others to explore the possible involvement of endorphins in pregnancy and parturition. We employed the opioid sensitive preparation, the guinea pig ileum, to measure opioid sensitivity in three groups of adult guinea pigs: non-pregnant, near parturition, and a few weeks after parturition. Elevated normorphine EC 50s occurred near the time of parturation. At 14–16 days post-partum, the EC 50s for normorphine had returned to non-pregnant control levels. These data point to the activation of endorphin systems at the time parturition.