This study was designed to compare the dose-related effects of U-50,488H on the cortical electroencephalogram (EEG) and behavior, following either saline or naloxone pretreatment. Adult female Sprague-Dawley rats were implanted with chronic cortical EEG and temporalis muscle electromyographic (EMG) recording electrodes and with permanent cannulae in the external jugular vein. U-50,488H injection produced initial "psychotomimetic-like" EEG and behavioral effects for about 10-20 min, followed by a predominance of behavioral stupor and associated EEG slow-wave bursts for about 20 min. Naloxone pretreatment completely antagonized the behavioral stupor and associated EEG slow-wave bursts. However, the "psychotomimetic-like" effects were not antagonized by naloxone pretreatment. Thus, dissociation between naloxone-sensitive and naloxone-insensitive effects of U-50,488H was demonstrated.