The present study was undertaken to define the relationship of urinary anion gap (UAG = sodium plus potassium minus chloride) to urine ammonium concentration in preterm and full-term neonates with spontaneous and NH4Cl-induced metabolic acidosis. Studies were performed in 10 premature infants (mean birth weight: 1,618 g, gestational age: 30.8 weeks) weekly for 6 consecutive weeks, in 11 full-term neonates (mean birth weight: 3,085 g, gestational age: 38.6 weeks) on the 7th day of life and in 25 older control infants (mean age: 6.5 months, body weight: 6,802 g) before and after NH4Cl-loading test. Blood acid-base parameters, plasma electrolyte concentrations, urine pH, ammonium, titratable acidity, bicarbonate, net acid and electrolyte concentrations were measured, UAG calculated. It was demonstrated that the significant reduction in plasma total CO2 content induced by NH4Cl administration and the subsequent elevation in urinary ammonium concentration was associated with some decrease of UAG in each group. In premature infants there was no relationship between urinary ammonium excretion and UAG during the first 2 weeks of life while from the 3rd week onward a significant negative correlation could be demonstrated. In one-week-old full-term neonates UAG also tended to decline with increasing ammonium concentrations; this relationship, however, proved to be insignificant. In the older control infants urinary ammonium excretion was found to correlate negatively to UAG. It is concluded that due to marked dynamic changes in unmeasured ionic composition of neonatal urine UAG is not a valuable index of urinary ammonium excretion in newborn infants during the first weeks of life.