In 14 very-low-birth-weight infants appropriate for gestational age the serum concentrations of urea and the excretions of urea and ammonium in the urine were studied on the 10th, 21st, and 42nd day of postnatal life. All infants were fed with a human milk preparation (6 g human milk lyophilisate per 100 ml fresh human milk). The enteral feeding could be started on the first day of life so that protein intakes were not different on the three study days (2.94 vs. 2.98 vs 3.13 g/kg.d). The renal excretion of urea increased significantly from the 10th to the 42nd day of life (1.2 +/- 0.13 vs 2.1 +/- 0.24 mmol/kg.d) whereas the excretion of ammonium decreased during the same time (0.53 +/- 0.06 vs 0.29 +/- 0.05 mmol/kg.d). There was a significantly negative correlation between the excretion of urea and of ammonia on all study days. The weight gain was sufficient for the given protein intakes (18.9 +/- 3.4 g/kg.d). The results indicate a transient limited capacity of the urea synthesizing system in the liver of very-low-birth-weight infants during the first weeks of life which should be considered if metabolic responses to protein intake are studied and evaluated in these infants.