The latency to tail-flick response in the rat was significantly prolonged by cerebroventricular infusion of 1.0 microgram of somatostatin (SRIF) and more so with 10.0 microgram. The D-tryptophan analog was less effective than native SRIF. Pretreatment with naloxone eliminated analgesia but not seizures induced by SRIF. Recording of the EEG activity enabled determination of the specific state of the sleep-waking cycle in which the repeated tail-flick responses were tested: latency was generally longer in both control and test animals when tail immersion was performed during the state of sleep or drowsiness rather than during the awake state. Although animals receiving SRIF were less likely to fall asleep between subsequent test trails, the average latency was actually longer than after control saline infusion when the animals slept more. SRIF, unlike other releasing factors and peptides tested, showed significant activity in an opiate radioreceptor assay. The blockade of SRIF action by naloxone pretreatment, along with binding of SRIF to opiate receptors in vitro, suggest opiate receptors to be involved in the mediation of analgesia observed in present study.