Erythrocytes were analysed for adenosine triphosphatase (total and sodium-potassium activated) activities, concentrations of sodium and potassium ions and ATP in normal individuals from birth to adulthood age. A decrease of Na+, K+-ATPase activity and an increase of the sodium concentration was seen in infants aged 1-3 and 4-8 months. A decrease of Na+, K+-ATPase was seen during puberty. A correlation was seen between the Na+, K+-ATPase activity and the Na+-K+ ratio. Different influences which might lead to the observed changes in Na+, K+-ATPase activity and erythrocyte sodium concentration are discussed.