Liver volume and the clearances of antipyrine and indocyanine green have been measured before and after administration of phenobarbitone (180 mg/day) for 3 weeks to ten healthy subjects. The measurement of liver volume by an ultrasound scanning technique yielded reproducible results which were consistent with predictions of liver size by allometric methods. Before phenobarbitone, antipyrine clearance correlated with liver volume, but there was no correlation between indocyanine green clearance and liver volume. Phenobarbitone administration increased the clearance of antipyrine significantly by 90 +/- 14% but there was no significant change in indocyanine green clearance or liver volume. After phenobarbitone the correlation between antipyrine clearance and liver volume persisted. There was no correlation between indocyanine green clearance and liver volume. These results suggest that in non-medicated subjects some of the difference in antipyrine clearance is due to difference in functional hepatic parenchymal mass and that phenobarbitone increases the drug metabolising capacity per unit of hepatic mass but not total liver size.