Here we report that a single administration of naloxone to conscious rats produces no significant increase in oxytocin release, but when repeated 3-4 days later results in a large release of oxytocin. Plasma oxytocin concentrations were measured in conscious and urethane-anaesthetized rats pretreated with naloxone or isotonic saline on Day 1. On Days 2, 3 or 4, a second dose of naloxone was given, producing an increase in oxytocin secretion in naloxone-pretreated groups (P < 0.05 vs. controls) on Day 3 and 4, but not on Day 2. The specificity of the opioid antagonist supersensitivity was determined by injection of the kappa-antagonist nor-binaltorphimine (nor-BNI). Pretreated rats (naloxone, saline or nor-BNI, Day 1) received an additional acute nor-BNI injection (Day 4) which increased plasma oxytocin concentration in the three groups. However, this increase was higher in naloxone-pretreated rats with no differences between the nor-BNI- and saline-pretreated animals. Measurements of electrical activity of single supraoptic nucleus oxytocin neurons and of plasma oxytocin concentration (Day 4) showed that naloxone modestly enhanced the responsiveness of oxytocin neurons to cholecystokinin (CCK) in naloxone-pretreated rats (by comparison with saline-pretreated rats), but had only a small effect on basal firing rate that did not differ between naloxone-pretreated rats and saline-pretreated rats. To investigate whether naloxone-pretreatment modified the effect of morphine on CCK-induced oxytocin release, on Day 4 CCK was injected i.v. with or without morphine. Morphine at a dose of 0.1 mg/kg did not affect CCK-induced oxytocin release, whereas 1 mg/kg of morphine blocked this release in both saline- and naloxone-pretreated rats. The results suggest that naloxone induces opioid antagonist supersensitivity on oxytocin secretion, mainly by up-regulating kappa-opioid mechanisms on oxytocin nerve terminals in the posterior pituitary.