In unanesthetized paralyzed rats, i.v. ethanol administration (0.5-2.0 g/kg) increased (by 30-120%) the firing rate of dopaminergic (DA) neurons in the substantia nigra, pars compacta. Doses of 4.0 g/kg or higher produced an initial stimulation followed by a long-lasting inhibition of firing. On the contrary, in rats anesthetized with halothane (2.5% v/v in air) or with chloral hydrate (400 mg/kg), doses of ethanol up to 2 g/kg failed to activate DA neurons, while a dose of 4 g/kg inhibited neuronal firing without the initial stimulant response. In unanesthetized-curarized rats, the i.v. administration of either chloral hydrate (100-400 mg/kg) or pentobarbital (10-40 mg/kg) or the inhalation of halothane (0.5-2.5% v/v in air) produced a dose-dependent increase in the firing rate of DA neurons. However, the maximum increase produced by these anesthetics was less pronounced and shorter lasting than that produced by ethanol.