In 30 very low birth weight (VLBW) infants, fed a commercial formula at 200 ml/kg/24 hr to provide 160 cal/kg/24 hr, hyponatremia (plasma Na+ less than 130 mEq/liter) occurred in 23 patients (14 appropriate for gestational age (AGA), 9 small for gestational age (SGA)) between the ages of 2 and 6 weeks. In five infants the hyponatremia recurred even after adequate correction of the deficit. Calcium supplementation given to 14 of the infants did not affect the incidence or severity of hyponatremia. In AGA infants, the hyponatremia was more severe than in SGA babies. Hyperkalemia (plasma K+ greater than 5.5 mEq/liter) was more common in AGA than in SGA infants (16/17 AGA, 7/13 SGA). Decrease in mean plasma chloride concentrations was proportionate to the decrease in mean Na+. Urinary Na+ averaged 1.0 mEq/kg/24 hr and was equal between groups during the first balance at a mean age of 18 days. In subsequent balances it appeared to decrease more rapidly in noncalcium-supplemented than in calcium-supplemented infants, but the difference was not significant (P less than 0.1). Fecal excretion of Na+ did not differ between groups. Symptoms were unrelated to the degree of hyponatremia or hyperkalemia and were nonspecific. Hyponatremia in AGA infants occurred in 50% of instances when growth was less than or equal to 0.75 cm/week, whereas it occurred in only 13.5% of infants when growth exceeded 1 cm/week (P less than 0.01). A similar trend in SGA infants was not statistically significant (P less than 0.2).