The effects of beta-endorphin and of morphine SO4 (0.5 microgram and 2.0 microgram, respectively, injected intraventricularly) upon the sleep-wakefulness behavior of cats were examined. Both agents produced insomnia. Deep slow wave sleep was sharply inhibited, and rapid eye movement (REM) sleep was entirely suppressed. Light slow wave sleep, occurring in brief, isolated episodes, became the most abundant stage of sleep. The nuchal electromyogram was markedly increased after both agents. Naloxone (100 microgram/kg), injected subcutaneously 30 min before beta-endorphin or morphine SO4, entirely reversed these agents' effects on the two stages of slow wave sleep, and antagonized the exaggerated electromyogram. But naloxone did not counteract the REM-suppressant effect of either beta-endorphin or morphine SO4. Total sleep time reverted towards control values after naloxone pretreatment, but not entirely; the difference may be due to the persistent deficit of REM sleep. The data may indicate an involvement of an inner opioid in the regulation of sleep and wakefulness in the cat, and may point to a role for more than one endorphin receptor in the effects of opioids on the states of vigilance in cats.