Studies with a new instrument show that blood ethanol concentrations in rats and humans can be estimated by measurement of ethanol vapor above the skin. After intravenous bolus administration of ethanol (1 g/kg) to rats a novel device based on the Figaro sensor was placed above the animal's abdomen. Plasma and skin vapor ethanol concentrations, analyzed by gas chromatography and sensor, respectively, declined in parallel (r= 0.96). In healthy human subjects, plasma and skin vapor concentrations, measured on the palm, also declined in parallel after intravenous ethanol infusion (1 hr, 0.5 g/kg), r= 0.99. In 10 alcoholic liver disease outpatients attending clinic in whom plasma ethanol concentrations ranged from 32–304 mg/dl, the correlation of plasma ethanol determined directly by gas chromatography and indirectly by skin vapor analysis was slope = 0.93, intercept = 1.8, r= 0.94. In controlled studies, skin vapor measurements are comparable with breathalyzer determinations; they may be performed in situations where breathalyzer measurements are inconvenient or where continuous monitoring is desirable.