The motor effects produced by intraventricular infusions of morphine were compared to the effects of etonitazene. Despite the similarity in the peripheral actions of these drugs, motor effects of central infusions differed dramatically. Intraventricular morphine infusions resulted in explosive motor behavior whereas etonitazene produced extreme muscular rigidity. The periaqueductal grey (PAG) has been proposed as the substrate of morphine-induced explosive motor behavior. However, considerations of the dose of morphine and the mobility of this drug in tissue suggests that sites other than the PAG may also be involved in explosive motor behavior.