The galactose elimination capacity, a measure of the functional liver cell mass, and liver volume were measured in 50 normal subjects of five different age groups (less than 50, 51 to 60, 61 to 70, 71 to 80 and greater than 81 years). The volume of the liver was evaluated by ultrasonography. All subjects had normal routine liver function tests and no history of liver disease. Galactose elimination progressively decreased from 3.05 +/- 0.58 (S.D.) mmoles per min in younger subjects to 1.83 +/- 0.24 mmoles per min in subjects over 81 (p less than 0.00003), without any change in the apparent volume of distribution of the sugar. Similarly, the estimated volume of the liver decreased from 110 +/- 14 units to 75 +/- 13 units with increasing age (p less than 0.0002). Both galactose elimination capacity and the estimated liver volume inversely correlated with age (r = -0.728 and r = -0.579, respectively) whereas a positive correlation was observed between galactose elimination and the estimated liver volume (r = 0.520). Part correlation analysis confirmed that age, when entered in a multiple regression already containing body weight and estimated liver volume as independent variables, had a significant effect on liver function, whereas no significant independent effect of liver volume was present. Both age and body weight had a significant independent effect on the estimated liver volume. The maximum functional capacity of the liver, measured by galactose elimination, is reduced in the elderly. Although several factors may play a role, our data suggest that aging is associated with a slight decline in the intrinsic metabolic activity of the hepatic parenchyma.