This study tested the hypothesis that concurrent ethanol administration attenuates the hypotensive effect of clonidine. Four groups of spontaneously hypertensive rats matched for baseline systolic pressure and body weight were randomly assigned the following treatments: (1) water (control), (2) ethanol, (3) clonidine, and (4) ethanol plus clonidine for 13 weeks. Ethanol was provided in the drinking water as 5% for 1 week, 10% for the next 2 weeks, and 20% from week 4 to 13. Starting from similar baseline systolic blood pressures, the blood pressure of the control group increased 10 to 15 mm Hg over the 13-week treatment period. A similar rise in systolic blood pressure occurred in ethanol-treated rats despite a drastic (40% to 50%, P<.05) reduction in fluid intake. Clonidine (300 p.g/kg per day) caused a smaller and shorter reduction in fluid intake. The fluid intake of the combined treatment group was similar to that of the ethanol group. Either treatment caused a significant and additive reduction in body weight gain. Treatment-related mortality (20%) occurred only in the combined treatment group by the 12th week. Clonidine elicited a slowly developing hypotensive response (P<.05) that started 2 to 3 weeks after treatment was initiated I n a previous study we showed that short-term ethanol administration elicited an immediate reversal of the hypotensive response to clonidine in conscious, unrestrained spontaneously hypertensive rats (SHR).1 This adverse effect of ethanol seems to be targeted against centrally acting hypotensive drugs. In support of this view is the finding that a similar hemodynamic interaction occurs between ethanol and another centrally acting drug, guanabenz.2 The hemodynamic interaction is dose related 2 and involves ethanol doses that are likely to be consumed clinically and lead to blood ethanol concentrations compatible with low to moderate intoxication. Interestingly, in the dose range used in these studies, 2 This latter effect may explain the immediate reversal by ethanol of the hypotensive response of centrally acting drugs that is the consequence of an inhibition of central sympathetic tone. From the Department of Pharmacology, East Carolina University School of Medicine, Greenville, NC.Correspondence to Abdel A. Abdel-Rahman, PhD, Department of Pharmacology, East Carolina University School of Medicine, Greenville, NC 27858.© 1994 American Heart Association, Inc.and lasted throughout the treatment period. Ethanol abolished the hypotensive effect of clonidine and resulted in blood pressure values that were not significantly different from those of the control or the ethanol groups. Blood ethanol concentration was similar in the presence or absence of clonidine (5.5±1.9 versus 6.5 ±3 mmol/L). We investigated whether long-term ethanol administration attenuates the hypotensive response elicited by centrally administered clonidine. The dose-response curve depicting the hypotensive responses to intracisternal clonidine in the ethanol-treated group was significantly shifted upwar...