Previous studies have shown that hepatic function, quantitatively measured by dynamic liver function tests, progressively declines with ageing. Urea synthesis is a specific process taking place in the liver; a reduced urea synthesis in response to a protein-rich metal has previously been demonstrated in the elderly, but the process has never been standardized in relation to amino acid supply. We measured the hepatic conversion of alpha-amino nitrogen into urea nitrogen in response to alanine infusion in 32 subjects, with normal routine liver and renal function tests and without evidence of previous hepatic disorders, belonging to three different age-groups (< or = 55 years, 56-70, > or = 71). The functional hepatic nitrogen clearance was reduced on average by 20% in subjects aged 56-70 years, and by 30% in subjects over 70 years old in comparison to the age-group under 55 years (ANOVA: P = 0.0001), and significantly correlated with age (r = -0.684). No sex differences were observed on the effects of age on hepatic clearance. Also, liver volume, measured by ultrasonography, was reduced with advancing age, but the age-related decrease in hepatic nitrogen conversion was not primarily dependent on decreased liver volume. The measurement of functional hepatic nitrogen clearance has already been validated as a quantitative liver function test in clinical hepatology. In keeping with previous studies, the age-related decline in hepatic nitrogen conversion points to a decreased functional capacity of the ageing hepatic parenchyma.