The effect of electrolytic lesions in the median and dorsal raphe nuclei was tested on acute tolerance development to ethanol and pentobarbital in the rat, as measured by motor impairment on the moving belt test. Acute tolerance to ethanol (1.7 g/kg, IP) or pentobarbital (17.5 mg/kg, IP) was monitored at 12.5, 25, or 50 min in separate subgroups tested only once each. One week of recovery was allowed between ethanol and pentobarbital tests. Median raphe lesions delayed the development of acute tolerance, whereas dorsal raphe lesions produced a negligible effect. These results were seen with both ethanol and pentobarbital. The mesolimbic 5-HT pathway from the median raphe nucleus is important in the development of acute tolerance to ethanol and pentobarbital, as was shown to be the case previously for chronic tolerance.