ABSTRACT.Energy and protein quantity and quality in the diet are factors regulating the rate of growth in the preterm infant. In the present study twenty infants 31–36 weeks of gestational age were fed with two identical test formulas which vried only in the content of energy. One formula (F‐81) contained 3.1 g protein (60 bovine whey: 40 bovine caseins) and 81 kcal per dl. The second formula (F‐94) contained the same amount of protein but 94 kcal per dl. At an intake of 150 ml/kg/d the infants received 4.6 g protein/kg/d and either 121 or 141 kcal/kg/d. The infants on F‐94 had a significantly higher rate of weight gain, but growth of length and head circumference was equal in the two groups. Significant differences were found in the plasma concentrations of glutamine and alanine between the two feeding groups. The other plasma amino acids were not statistically different in the two groups of infants. Urine excretion of threonine, serine, glycine, alanine, histidine, tyrosine, glutamine and cystathionine was significantly increased in the high caloric, F‐94‐group. The results indicate that increasing the caloric intake above 120 kcal/kg/d in preterm infants on a relatively high protein intake does not increase linear growth but does produce increased weight gain. The biochemical results provide indirect evidence that this weight increase is the result of increased fa