Ethanol was administered to 5 healthy volunteers by intravenous infusion to maintain a constant blood-alcohol concentration (BAC) of about 0.8 mg/ml (17.4 mmol/1) for 3-4 hr. Before starting the infusion and at 15 min. intervals thereafter, we analysed end-expired air by a highly sensitive gas chromatographic method. The presumed concentrations of ethanol and methanol in blood were estimated indirectly from measurements in the breath. The liquid/air partition coefficients of methanol were determined for whole blood, plasma, water, and cornoil when dilute solutions were equilibrated at 34° and 37°. The results at 37° were 2709 ± 165, 3400 ± 86, 2948 ± 221 and 44.5 ± 6.3 for whole-blood/air, water/air, plasma/air and corn-oil/air respectively. During infusion of ethanol, the concentrations of endogenous methanol in blood rose from being between 0.4-0.8 mg/1 to reach between 1.2-3.4 mg/1 with a substantial inter-subject variation in rate of increase. Our results confirm the existence of endogenous methanol in human blood and breath and demonstrate that the concentrations present can rise to abnormally high levels when the body is flooded with exogenous ethanol.