Gamma hydroxybutyrate (GHB) was administered to adult and prepubescent rhesus monkeys intravenously in varying dosages while an electroencephalogram (EEG) was recorded from scalp electrodes and the body core temperature was monitored. Blood and cerebrospinal fluid samples were assayed for gamma hydroxybutyrate. GHB produced a trancelike stupor in all the monkeys, associated with marked EEG changes and hypothermia. There was a striking age specificity in that prepubescent rhesus monkeys responded to a lower threshold dosage, had a higher incidence of myoclonic jerking, and showed characteristic EEG changes not seen in the adult animals. The EEG-behavioral changes paralleled the hypothermia. There was good correlation between the serum levels of GHB and the EEG-behavioral effects. These studies suggest that the GHB-treated monkey may have utility as a petit ma1 seizure model.