We have investigated the role of endogenous opioid peptides in the release of oxytocin (OT) in response to breast feeding and breast stimulation in humans. Five breast feeding women were studied on two separate occasions within 4 weeks of delivery. Saline or naloxone, 4 mg bolus and 6 mg/h, was administered intravenously, in random order. Blood samples were taken at regular intervals. In the saline-infused group OT rose from a baseline of 1.1 +/- 0.1 pmol/l (mean +/- SEM) to a peak of 7.0 +/- 0.9 after 6 min, and in the naloxone-infused group from 1.0 +/- 0.1 pmol/l to 5.8 +/- 1.3 (P less than 0.05). There were no significant differences between the two groups at any time point. Plasma vasopressin (AVP) did not change. In the second study six women in the luteal phase of the menstrual cycle were investigated on two occasions at least 48 h apart. They were similarly infused with either naloxone or saline in random sequence. A mechanical breast pump provided breast stimulation. In saline-infused women OT levels rose from a baseline of 1.0 +/- 0.1 pmol/l (mean +/- SEM) to a peak of 3.0 +/- 1.1 (P less than 0.05) after 6 min, and in naloxone infused women from 1.1 +/- 0.1 pmol/l to 3.0 +/- 1.4 (NS). There were no differences in OT between the groups. AVP did not change. We conclude that endogenous opioid peptides do not modulate OT release during breast feeding or breast stimulation in women.