Metabolism in erythrocytes was studied in a group of premature and full-term, newborn, healthy infants by assay of the following enzymes: glucose-6-phosphate dehydrogenase, 6-phosphogluconic dehydrogenase and aldolase. Concentrations of fetal hemoglobin and stability of GSH were also determined. The studies indicate increased metabolic activity in the erythrocytes of young subjects. The pentose phosphate pathway appears to be very active at birth with gradual decline, to values found for adult subjects, in the second 6 months of life. GSH is unstable in newborns less than 3 days of age despite high levels of activity of G6PD in the same subjects.