In order to examine the effects of blockade of the opiate-receptor during gestation and parturition, pregnant rats were implanted with subcutaneous minipumps, loaded with either naloxone (100 or 30 mg/ml) or saline, released at a constant rate for 7 days. It was found that neonatal mortality was significantly increased in the group that received naloxone 0.1 mg/h from day 17 of pregnancy, compared to saline controls. Body weight increase was slightly retarded by administration of naloxone 0.03 mg/h, starting day 17. At the age of 40 days, the groups exposed to naloxone 0.03 mg/h during late gestation showed a significant analgetic response to morphine 5 mg/kg, in contrast to saline controls, when tested with the hot-plate technique. The results suggest a role for endorphins during parturition and development.