In 14 very low birth weight (VLBW) infants appropriate for gestational age the absorption and retention of nitrogen and the energy balance were studied on the 8th day and the 6th week of postnatal age. All infants were fed with a human milk formula composed by fresh mother's milk enriched with 6 g freeze-dried human milk/100 ml. The absorption rates of nitrogen and fat increase significantly with postnatal age but even on the 8th day of life they are on high levels (nitrogen absorption: 86.9 +/- 3.2% of intake; fat absorption: 85.7 +/- 4.3% of intake). Despite the higher nitrogen absorption during the 6th week of life the excretion of total nitrogen is lower in comparison to the 8th day of life. Similarly, the percentages of alpha-amino-nitrogen and of ammonium-nitrogen of the total nitrogen excretion in the urine decrease whereas the percentage of urea-nitrogen increases with postnatal age. The results indicate a postnatal development of both the digestive, as well as the metabolic capacity. The postnatal adaptation of the metabolic capacity to utilize nitrogen seems to be the stronger limiting factor for nutrition than the functional capacity of digestion and absorption. Thus, metabolic monitoring is essential for the individual nutrition of VLBW-infants during the first weeks of postnatal life. Urine analysis data are very sensitive for evaluating the metabolic state of these infants.