Two groups (n = 20 each for pill and no pill groups) of young female social drinkers were tested with an acute ethanol dose (0.52 g/kg) as a 20% solution in water. Ethanol pharmacokinetics and acetaldehyde were repeatedly assayed from breath by a gas chromatograph. Cardiovascular indices were concurrently recorded. Groups did not differ on ethanol pharmacokinetics. However, the group on birth control pills reached a significantly higher acetaldehyde concentration than did the group of normally cycling females not on birth control pills (3.26 ng/ml vs. 1.45 ng/ml). Further, the pill (P) group showed significantly greater vasodilation 20 min postdrink as indexed by ear lobe plethysmography than did the no pill (NP) group (P = 160.3% vs. NP = 113.9%). Results are consistent with the interpretation that female sex steroids modulate aldehyde dehydrogenase function. Practical implications are that females with elevated steroids (either pregnant or on birth control pills) may be at greater risk for toxic effects of ethanol consumption.