• Studies of arterialized capillary blood gases and acid-base values were performed on 712 normal persons who were between the ages of 29 days and 24 years. The results were divided for clinical use into six groups by age; linear regression lines were constructed for better understanding of the development of blood gas levels and the acid-base balance. The blood pH essentially remained unchanged throughout the study period. The oxygen and carbon dioxide pressures, base excess of extracellular fluid, and buffer base and bicarbonate levels are lower in infants than in children and adults; adult values are reached at approximately 7 to 12 years of age. The probable explanation for the lower values in infants is discussed. The effects of crying on the composition of blood gases are mentioned.