In rats, lactation is accompanied by changes in the pattern of oxytocin (OXY) release associated with the onset of reflex milk ejection during suckling. The present study attempted to identify further differences between OXY release in virgin and lactating rats. Basal levels of plasma immunoreactive (ir)-OXY and the OXY responses to both water-deprivation and intraperitoneal administration of hypertonic saline were lower in lactating than in virgin female rats. Subcutaneous administration of morphine (0.01-10.0 mg/kg) was followed by a dose-related reduction in plasma ir-OXY levels in virgin but not lactating rats. The OXY response to naltrexone (3 mg/kg) was similar in both types of animal. Lactation-associated changes in the control of metabolism and/or release of OXY may involve changes in neurohypophysial morphology but do not involve alteration of tonic endogenous opioid influences on OXY release.