Seven very low birthweight (VLBW) infants, small for gestational age (SGA), with moderate intrauterine growth retardation and 7 VLBW-infants, appropriate for gestational age (AGA), fed breast milk fortified with 6 g freeze-dried human milk per 100 ml were studied on the 8th, 21st and 42nd days of life. The protein intake on the study days varied between 2.68 and 3.61 g/kg/day in the SGA-and 2.69 and 3.75 g/kg/day in the AGA-infants. Serum concentrations of total bile acids (BA) and the renal excretion of total nitrogen (TN) as well as alpha-amino-nitrogen (AAN) were measured in all infants on each study day. On the 8th day of life a mean protein intake of 3.2 g/kg/day resulted in higher serum concentrations of BA as well as in a higher renal excretion of TN and AAN in the SGA-infants when compared to the AGA-infants. On the 21st day of life these differences were smaller and only the serum concentration of BA and the renal excretion of AAN were still significantly higher in the SGA-infants. On the 42nd day of life only serum concentrations of total BA were elevated in the SGA-infants when compared to that in the AGA-infants. The observed metabolic differences between moderately SGA-and AGA-infants related to protein and bile acid metabolism diminished during the first weeks of life. The present data suggest that when nutritional management of VLBW-infants is planned, differences in metabolic capacities must be considered and protein intake should be increased with caution and in accordance to the individual metabolic situation of the infants during the first weeks of life.