γ-Hydroxybutyrate (GHB) was examined for an application as an activating agent of clinical electroencephalogram (EEG). As the control group, 100 normal healthy subjects were administrated GHB by intravenous injection (20-25 mg/kg of body weight, rapidly infused, 1,000 mg/min approx.). Eighty five out of 100 subjects went to sleep within 6-18 minutes, 36 subjects showed natural sleep like pattern, 25 subjects showed a 'prehypnagogic theta bursts' and 24 subjects presented 'prehynagogic theta' at the stage of suppressed alpha waves. The patients with organic brain disorder (tumor, CVA or others) who did not show any difinitely abnormal waves under routine EEG examination were surveyed by GHB activation as the same manner as the control group. Fourteen out of 79 patients (17.8%) who were normal by routine EEG showed marked abnormality (focal or hemispheral slow activity), 22 patients (27.8%) who showed equivocal or borderline abnormality in routine EEG revealed remarkable abnormality and 21 patients (26.6%) who were minimal EEG abnormality showed exaggerated abnormality. As regards the correlation between the localization of brain lesions and activating effects, the hemisphere lesion showed the most activity, while deep or intratentorial lesion showed less activity. The patients with epileptic attacks were examined as the same way. Twenty one out of 108 patients (19.4%) who did not show epileptic abnormal pattern of routine EEG showed various kinds of epileptic descharges and 17 patients (15.7%) who showed a few epileptic abnormality under routine EEG revealedd remarkable epileptic abnormality by GHB activation. The most marked activating effect was induced in case of psychomotor epilepsy. Side effects of GHB administration were mild general lassitude, headache, nausea and some others. These symptoms were, however, disappeared completely within 10 to 20 minutes. No side effect was observed in 64.6 % of the all cases (124 cases out of 187). It was concluded that GHB is an ideal drug for activation of clinical EEG, especially for the diagnosis of brain organic disorders that showed normal or equivocal pattern under routine EEG examination.