Acetaldehyde is the first and principal metabolite of ethanol administered systemically. To its rise in blood, after administration of disulfiram, is ascribed the aversive reaction that should discourage alcoholics from drinking. In the present study, we sought to determine the effect of acetaldehyde on the electrophysiological properties of dopamine (DA)-containing neurons in the ventro tegmental area (VTA) of rats in vivo. Intravenous (i.v.) administration of acetaldehyde (5-40 mg/kg) readily and dose-dependently increased the firing rate, spikes/burst, and burst firing of VTA neurons. Ethanol (250-1000 mg/kg/i.v.) administration produced similar increments in electrophysiological parameters. In addition, a second group of rats was pretreated with the alcohol-dehydrogenase inhibitor 4-methylpyrazole (90 mg/kg) intraperitoneally (i.p.), and ethanol and acetaldehyde were administered i.v. at the same doses, 48 h later. In this group, ethanol effects were drastically reduced and the firing rate, spikes/burst, and burst firing were not significantly altered. In contrast, acetaldehyde fully retained its capacity to stimulate electrophysiological indices. The results indicate that acetaldehyde produces electrophysiological actions on VTA neurons in vivo, similar to those produced by ethanol, and significantly participate in ethanol-induced increment in DA neuronal activity. These results also suggest that acetaldehyde, by increasing DA neuronal activity in the VTA, may significantly contribute to the centrally mediated positive motivational properties of ethanol, which would oppose the well-known peripherally originating aversive properties.