Acid-base equilibrium of arterialized capillary blood was determined on 139 normal infants between the ages of 3 months and 2 years. The mean and standard deviation of each component was as follows: pH 7.398 ± 0.027, base excess -3.2 ± 1.7 mEq/L, pCO2 33.8 ± 3.7 mm Hg, and plasma total CO2 content 21.1 ± 1.9 mM/L. When compared to normal adult values, all components except blood pH were reduced to a statistically significant extent. These data are proposed as definitive standards for normal acid-base equilibrium in infants of this age group.