In pre- and post-natal period nutrition can influence the function of many organs, including the kidneys. Intrauterine growth restriction and low weight at birth are associated with reduced nephron number, a risk factor for later cardiovascular and renal diseases. The development of such adult diseases may be favored, in animals, by additional risk factors, including postnatal overnutrition and/or rapid postnatal growth. In preterm infants, during the first weeks of life, high values of serum urea are presents due to immaturity of the renal function. Thus the urea cannot be used in the first weeks of life as a parameter to evaluate the adequacy of protein intake. In comparison with older infants, healthy preterm infants, fed on human milk and adapted formulas, show a lower renal solute load because the higher growth rate associated with a raised nitrogen and mineral retention rate. Preterm infants are vulnerable to disturbances of acid-base metabolism, with a predisposition to metabolic acidosis due to a transient age-related low renal capacity for net acid excretion.