Ethanol (0.6 gm/kg) was given orally as a cocktail to 6 healthy fasted volunteers (3 men and 3 women) on 4 separate occasions at 2300 hr. Subjects were recumbent and remained awake in 2 sessions while sleep was allowed in the other 2. An indwelling forearm venous catheter was used to allow blood sampling without disturbance of sleep, and 20 serial blood samples (2 ml) were drawn at appropriate intervals in the 8 hr after the cocktail. Blood ethanol concentration was determined by a head-space gas chromatographic technique and Michaelis-Menten parameters (Vm and Km) were estimated from data in the terminal elimination phase. Mean estimates were Vm awake = 0.214 +/- 0.009 mg/ml/hr (SEM), Vm asleep = 0.221 +/- 0.017, Km awake = 0.069 +/- 0.009 mg/ml, and Km asleep = 0.085 +/- 0.016. These values were slightly lower than literature reports for elimination in ambulant patients during the daytime. Mean differences in Vm and Km between the awake and asleep groups and between sexes were not significant (p greater than 0.05).