Microinjections of morphine in optimal doses into the medial thalamus (40 µg) and periaqueductal gray matter (10–20 µg) produce slow-wave sleep with abondant spindles, in addition to analgesia. Like analgesia, the sleep-inducing effect is blocked by naloxone (1 mg/kg i.v., l60 µg i.c). One may therefore conclude that the effect is related to the agonistic action of morphine on endorphin receptors. It is thus probable that the endorphins constitue a regulatory system acting on the medullary-thalamic sleep-inducing structures which generate the sleep spindles. At higher dose levels, injections of morphine into the same structures produce behavioral agitation resembling the dissociated REM sleep described by Jouvet and which is not blocked by naloxone. The agitation might be due to an indirect action of morphine, and therefore of endorphin receptors, acting on monoaminergic struct