Twenty premature low-birthweight infants were divided into two groups and assigned randomly to either a pooled human milk or to a cow's milk based infant formula feeding regimen. The protein intake was 2.0 g/kg/day in the human milk fed group and 4.4 g/kg/day in the formula fed group of infants. The concentrations of different metabolites were estimated at weekly intervals, and plasma amino acid analysis was performed biweekly on blood samples in the two groups of infants during the four-week study period. Formula milk fed infants had significantly lower fasting blood glucose levels and developed azotaemia, hyperaminoacidemia and metabolic acidosis in the early weeks of postnatal life. Blood lactate and plasma free fatty acid concentrations did not change significantly in the two groups during the study. No significant differences were found in the rate of weight gain between the two groups of infants, although formula fed infants regained their birthweight more slowly than human milk fed infants. High protein formula feeding causes potentially unfavorable metabolic and amino acid imbalances in preterm infants in the early postnatal life.