This study was conducted to determine if plasma ethanol levels are altered as a result of smoking marihuana. Fifteen healthy adult male volunteers who used ethanol and marihuana on a casual basis participated in this study. Subjects were randomly assigned to one of three groups: placebo, low-dose, or high-dose marihuana. The marihuana dose was held constant and each subject drank three different doses of ethanol on 3 separate days spaced at least 1 week apart. Subjects drank either placebo or ethanol at doses of 0.35 g/kg (7.60 mmol/kg) or 0.70 g/kg (15.19 mmol/kg). Thirty minutes after drinking they smoked either a placebo marihuana cigarette, or one containing either 1.26% or 2.53% delta 9-tetrahydrocannabinol. Plasma ethanol levels rose sharply after the 0.7 g/kg dose and peaked at 50 minutes after drinking began (78.25 +/- 4.95 mg/dl). When subjects smoked the high-dose marihuana cigarettes after the 0.7 g/kg dose of ethanol, peak plasma ethanols levels were only 54.80 +/- 8.32 mg/dl at 105 minutes after drinking began. These alterations in plasma ethanol levels paralleled a reduction in the duration of ethanol- and marihuana-induced subjective effects after high doses of both drugs. These data suggest that marihuana may alter ethanol bioavailability.