SummaryIn this investigation comprising 516 neonates the frequency of metabolic acidosis from day 5 to day 21 of postnatal life is reported. 334 term and 131 preterm appropriate‐for‐gestational age (AGA) infants and 51 small‐for‐gestational age (SGA) infants were studied. The incidence of metabolic acidosis occurring after the 5th day of postnatal life was in each group of infants 4.79%, 20.61% and 11.76% respectively.Furthermore, the infants were divided into three groups according to the amount of protein and solute content of the formula consumed. An increasing incidence of metabolic acidosis with increasing dietary protein intake was observed, especially among preterm infants. In the group of small‐for‐gestational age infants a similar increase with a higher protein and solute content of the formula was registered. However, this augmented incidence was related to the gestational age, rather than to b