Although moderate alcohol consumption is postulated to enhance theophylline metabolism, to our knowledge, the effect of casual nocturnal alcohol consumption has not been assessed. Eight normal volunteers, at the same time, one week apart, were given whiskey, 3 ml/kg of body weight, on one evening and on the alternate evening an equivalent volume of water. Each evening was followed by assessment of the pharmacokinetics of fast-release bibtheophylline, 5 mg/kg, taken the next morning at 9 AM. Alcohol taken the night before caused a statistically significant decrease in plasma clearance together with an increase in elimination half-life of 33 percent with consequent increases in plasma concentrations. These findings have potentially important implications for the accuracy of outpatient monitoring of serum theophylline and might explain intermittent toxic reactions occurring in some patients.