The half-life and metabolic clearance rate (MCR) of antipyrine and phenytoin were determined in 14 young (mean age: 28.8 +/- 8.3 (SD) years] and in 14 elderly [mean age: 83.5 +/- 7.1 (SD) years] subjects and correlated with liver volume, which was determined by ultrasonic scanning, to see if an age-dependent difference in drug metabolism could be explained by a reduced liver weight with age. The size of the liver was smaller in the elderly subjects even when related to decreased body surface. A significant decrease in serum albumin in the elderly compared with the younger group was also noted. The half-life of antipyrine was significantly longer in the elderly than in the younger group, 756 +/- 318 and 465 +/- 110 minutes, respectively, and the MCR was correspondingly lower in the elderly even when calculated per litre of live volume, 22.8 +/- 7.8 and 36.3 +/- 8.9ml/minute/L liver volume, respectively. No significant differences in the 2 age groups were found in half-life and total clearance of phenytoin, but a reduced free phenytoin clearance was demonstrated in the elderly (240 +/- 92ml/minute/L liver volume) compared with the younger (325 +/- 81ml/minute/L liver volume) group. No significant correlation was found between liver volume and the half-life of antipyrine and phenytoin. However, a significant correlation was demonstrated between liver volume and MCR of antipyrine as well as between total and free clearance of phenytoin. No correlation was found between the half-lives of the 2 drugs, while a significant correlation existed between the clearance values. It is suggested that the age-dependent reduction in drug clearance is due not only to a smaller liver volume, but it also a result of a reduced capacity of the liver microsomes per unit of liver in the elderly. With regard to age-dependent changes in drug metabolism, the protein binding of the actual drug has to be taken into consideration.