Plasma oxytocin concentration (shown on a log scale) in response to hyperosmotic stimulation and naloxone (5 mg/kg, i.v.) in virgin (A) (1.5 m NaCl; 4 ml/kg, i.p.) or pregnant (B) (2 mNaCl; 5 ml/kg, i.p.) rats pretreated with either l-NNA or isotonic saline. Oxytocin secretion was enhanced by pretreatment withl-NNA in virgin rats but not in pregnant rats. Means ± SEM; *p < 0.05 compared with basal; #p < 0.05 between groups.