In pithed rats, the intravenous administration of serotonin (3, 10, 30, 100, 300 and 1,000 micrograms/kg) produced a dose-dependent increase in blood pressure. This action of 5-HT was not changed by chronic ethanol intoxication (6.0 g/kg/day for 2 weeks). The pressor responses to serotonin were, in a dose-dependent manner, significantly reduced by acute ethanol administration (0.5, 1.0, 2.0 and 4.0 g/kg p.o.) and by ketanserin injection (0.03 mg/kg i.v.). However, ethanol (2.0 g/kg) did not amplify the dilator effect of serotonin in pithed rats pretreated with ketanserin (3.0 mg/kg i.v.). We also demonstrated that, in contrast to ketanserin (10(-7) mol/l), ethanol (0.05 mol/l) potentiated the serotonin-induced vasoconstriction of isolated rat tail arteries. These data suggest that the action of ethanol on the vasopressor effect of serotonin in pithed rats does not depend on its influence on serotonergic receptors in blood vessels.