This study investigated the effects of intrafetal morphine or naloxone administration on fetal and maternal plasma prolactin concentrations in the sheep during late pregnancy (117–143 days gestation). After intravenous morphine (3 mg/kg) there was a significant increase (p < 0.05) in fetal plasma prolactin concentrations which was sustained for 180 min post-injection. There was no significant effect of either gestational age (125–133 days compared to 134–143 days gestation) or repeated administration (up to 3 treatments) of morphine on the fetal prolactin response to morphine. Intrafetal administration of naloxone (3.8 mg/kg bolus + 9.9 mg/kg/60 min), blocked the fetal prolactin response to morphine. Maternal plasma concentrations of prolactin were significantly increased (p < 0.05) at 180 min after the intrafetal morphine bolus. When naloxone alone was infused, there was no change in fetal plasma prolactin concentrations, but there was a significant fall (p < 0.05) in maternal plasma prolactin from 25 min after the start of the naloxone infusion. Thus, acute administration of morphine is associated with fetal and maternal hyperprolactinaemia. Although the endogenous opioids do not appear to mediate basal prolactin secretion in the fetus, they may have a role in the control of prolactin release in the pregnant ewe during late gestation.