Previous reports from our laboratory have shown that ethanol elicits hypotension in female but not in male rats and that this effect of ethanol is estrogen dependent (El-Mass MM and Abdel-Rahman AA. Alcohol Clin Exp Res 23: 624–632, 1999; El-Mass MM and Abdel-Rahman AA. Clin Exp Hypertens 21: 1429–1445, 1999). In the present study, we tested the hypothesis that ethanol lowers blood pressure in female rats via upregulation of the inducible nitric oxide synthase (iNOS) in vascular tissues. The effects of pretreatment with N^G-nitro-l-arginine (NOARG; nonselective nitric oxide synthase inhibitor) or aminoguanidine (selective iNOS inhibitor) on hemodynamic responses elicited by intragastric (ig) ethanol were determined in conscious female rats. Changes in vascular (aortic) iNOS protein expression evoked by ethanol in the presence and absence of aminoguanidine were also measured by immunohistochemistry. Compared with control (water treated) female rats, ethanol (1 g/kg ig) elicited hypotension that was associated with a significant increase in the aortic iNOS activity. The hypotensive effect of ethanol was virtually abolished in rats infused with the nitric oxide synthase inhibitor NOARG, suggesting a role for nitric oxide in ethanol hypotension. The inability of ethanol to lower blood pressure in NOARG-treated rats cannot be attributed to the presence of elevated blood pressure in these rats because ethanol produced hypotension when blood pressure was raised to comparable levels with phenylephrine infusion. Selective inhibition of iNOS by aminoguanidine (45 mg/kg ip), which had no effect on baseline blood pressure, abolished both the hypotensive action of subsequently administered ethanol and the associated increases in aortic iNOS content. These findings implicate vascular iNOS, at least partly, in the acute hypotensive action of ethanol in female rats.