1. The effects of chronic oral administration of ethanol (EtOH, 80 mM daily during 15-18 days) on the modulation exerted by the endothelium on the contractions induced by phenylephrine (Ph) and clonidine (C) were studied in rat aortic rings with and without endothelium. 2. The maximal contraction induced by a 70 mM KCl depolarizing solution was similar in control and EtOH treated-rings. 3. EtOH pretreatment significantly enhanced the contractile response to Ph in vessels with intact endothelium, but did not significantly affect the response of endothelium-denuded rings. 4. This effect reflects a reduction of the modulation exerted by the endothelial cells on alpha 1-adrenergic vasoconstriction. 5. C did not contract control aortic rings with endothelium, but after EtOH treatment, the response reached about 20% of the maximal KCl-induced contraction. 6. EtOH pretreatment significantly enhanced the contractile response to C both in endothelium-denuded and intact aortic rings. 7. The results indicate that chronic EtOH consumption significantly potentiates alpha-adrenergic-induced contractions in rat aortic rings, probably through interference with the production and/or the release of EDRF.