An increased liver blood flow in rats and monkeys found following induction of the liver microsomal enzyme system by antipyrine and phenobarbitone, formed the basis of the present study in 7 volunteers. The total body clearance of antipyrine, gamma GT, and the urinary excretion of d-glucaric acid and 6-beta-hydroxycortisol were measured. Liver blood flow was estimated after an overnight fast under basal conditions using the 133Xenon inhalation technique. In addition, liver volume was determined by 7 independent investigations using a 99mTechnetium-sulfur-colloid scan of the liver. Afterwards, each volunteer received 1,000 mg antipyrine daily for 14 days and all measurements were repeated. After antipyrine administration the antipyrine half-life decreased significantly from 12.5 to 7.7 hours with an increase of the antipyrine clearance from 34.1 to 50.8 ml/min. In addition, glucaric acid, gamma-GT and 6-beta-hydroxycortisol were significantly increased. Liver blood flow increased from 36.8 ml/min/100 g to 50.9 ml/min/100 g liver (p less than 0.02). The liver volume showed a tendency to increase but was significantly higher in only three of the seven volunteers investigated. The mean liver volumes of 1483 g before and 1585 g after antipyrine administration were not significantly different. In contrast, total liver blood flow increased significantly from 590 ml/min before to 809 ml/min after enzyme induction (p less than 0.02).