In in vitro conditions ethanol dose-dependently contracts the isolated tail artery and aorta of the rat. In concentration 0.03 M ethanol did not change the perfusing pressure in isolated vessels, but potentiated the contracting action of serotonin. In concentration of 0.1 M ethanol did not change the sensitivity of blood vessels to serotonin, and in concentration of 0.3 M inhibited it. Statistically significant changes were observed only in tail artery. The tail artery isolated from the rat receiving a single dose of ethanol (2 g/kg) displayed decreased sensitivity to action of serotonin. Chronic administration of ethanol (6 g/kg/day for 14 days) did not change the serotonin-induced contraction of the isolated tail artery of the rat. The present data indicate that ethanol modifies the sensitivity of rat blood vessels to serotonin.