The response of infants and children to a prolonged administration of ammonium chloride was established. The children were divided into three groups: group A included the children aged 3 months to 2 years, who had been given 80 mEq ammonium chloride/m3/day for 5 days (n = 14); group B included those children over 2 years old who had 80 mEq ammonium chloride/m2/day (n = 13), and group C those children over 2 years old who received 100 mEq ammonium chloride/m2/day also for 5 days (n = 16). The urine was collected over a period of 24 h on the first, the third and the fifth days of the test. Urine pH was the same in the three groups. On the fifth day, a significant negative correlation of the excretion of titratable acidity with age could be demonstrated. Most of the days there was a significant decrease of the phosphorus excretion with age. This may be explained by the relative large amount of phosphate in the diet of younger children. The increase in ammonia excretion with age may be explained by the increase in supply of precursors to the tubulus. The rate of excretion of total hydrogen ion did not increase with age, the higher rate of excretion of titratable acid in the younger age group balancing their lower rate of ammonia excretion.