To establish nutritional management of low birthweight infants according to their individual metabolic situation, hepatocellular partial function was studied in 13 appropriate (AGA) and 11 small-for-gestational-age (SGA) low birthweight (LBW) infants during the first weeks of postnatal life. The concentrations of total bile acids and of alpha-amino-nitrogen in serum, the renal excretion of urea and ammonia and the renal excretion of15N after enteral administration of 3 mg15N-labeled methacetin/kg were measured. In comparison to AGA infants, SGA infants had elevated serum concentrations of total bile acids and of alpha-amino-nitrogen, decreased excretion of urea, increased excretion of ammonia in urine, and lower urinary15N-excretion after enteral administration of15N-labeled methacetin. The data suggest that hepato-cellular functions are influenced by intrauterine growth retardation resulting in a reduced metabolic capacity in SGA infants. The metabolic differences between SGA and AGA infants should be considered in the nutritional management of LBW infants.