Two hundred and eighty-two patients with birthweights below 2.0 kg were routinely screened for spontaneous development of maximum renal acid stimulation (urine-pH < 5.4). Sixty episodes in 53 patients of incipient late metabolic acidosis (urine pH < 5.4 on 2 consecutive days) were randomly allocated to oral therapy with 2 mmol/kg/day of either NaHCO3 or NaCl for 7 days. All 27 patients on NaHCO3 therapy, but only 15 from 26 patients on NaCl therapy, showed an increase in urine pH values, combined with a relatively high gain in body weight and a tendency to increased N-assimilation. Eleven patients on NaCl therapy showed persistent maximal renal acid stimulation on all 7 days with possibly lower weight gain and no clear change in N-assimilation. Thus, in patients with incipient late metabolic acidosis, NaCl therapy is not as beneficial as NaHCO3 therapy.