OBJECTIVE Modification of the inhibitory control of oxytocin secretion by endogenous opiates in late pregnancy may be one of the factors involved in the onset of labour. The interrelationships between exogenously administered opioids and oxytocin may demonstrate activity of this control mechanism.

This study was undertaken to investigate the effect of an opiate and an opiate antagonist on maternal oxytocin levels in late pregnancy.

DESIGN Patients were randomized to receive either morphine (5 mg), naloxone (2.4 mg) or sterile water (1 ml) intravenously.

PATIENTS Thirty women with singleton pregnancies greater than 36 weeks gestation who were not in labour were studied.

MEASUREMENTS Blood for peripheral oxytocin levels was sampled every 2.5 minutes for 15 minutes before and 15 minutes after administration of the assigned substance. Oxytoxin was measured by radioimmunoassay. Peripheral β-endorphin levels were measured at the beginning and end of the study.

RESULTS There was no significant change in the maternal oxytocin concentration after administration of either morphine, naloxone or sterile water compared to pretreatment levels. β-Endorphin levels did not change significantly, either from the beginning to the end of the study, or between groups.

CONCLUSION In late pregnancy exogenous opiates and opiate antagonists have no effect on maternal peripheral oxytocin levels.