1. An investigation was carried out to determine the effect of intracerebroventricular (I.C.V.) micro-injections of morphine on vasopressin (AVP) release in the urethane-anaesthetized rat. 2. Plasma AVP levels at different time intervals, following I.C.V. injection of 10-150 microgram morphine, were measured by radioimmunoassay. The effect of I.C.V. micro-injections of morphine on urine outflow was also studied in a group of water-loaded rats. 3. The vasopressin response to I.C.V. micro-injections of morphine was both dose- and time-dependent. High dose of 50 and 150 microgram morphine produced short latency stimulation of AVP release, followed by a fall. The low dose of 10 microgram morphine produced only a long latency inhibition. The most consistent response of I.C.V. injection of morphine was an inhibition of release. 4. Both stimulatory and inhibitory effects of morphine on vasopressin release were naloxone reversible and stereospecific. 5. I.C.V. micro-injections of morphine produced a dose-dependent rise in mean arterial blood pressure of short latency. Naloxone (0.5 mg/kg) completely abolished the rise seen with 10 microgram morphine and diminished the rise with 50 microgram. 6. Doses of 10 and 50 microgram morphine injected I.C.V. produced an immediate antidiuresis in water-loaded rats under urethane anaesthesia. 7. The vasopressin response to I.C.V. micro-injections of morphine is independent of the effects on the cardiovascular system and may involve different opiate receptor populations. The results also suggest the possibility that opiate receptors with different affinities for morphine may be responsible for the stimulatory and inhibitory effects of morphine on vasopressin release.