The oxidative and non-oxidative utilization of glucose was evaluated in human erythrocytes of different ages, separated by density gradient ultracentrifugation. Young red blood cells are able to metabolize 2.5 times more glucose than old ones; on the other hand, the amount of glucose utilized via the hexose monophosphate shunt does not show any age dependence. Glucose metabolism evaluated during in vivo ageing of a rabbit red cell population shows results very similar to those obtained for human cells. Metabolic stimulation of glucose utilization by high phosphate in both young and old human red cells increases glucose utilization by 40%. In the same way young and old erythrocytes were able to increase the amount of glucose metabolized via the hexose monophosphate shunt when an oxidative stimulus (methylene blue) was introduced. Human erythrocytes of different age possess similar abilities to transport glucose so that an age-dependent defect in glucose transport can be excluded. The ATP content of human and rabbit red blood cells, as a function of cell age, follows the decrease in glucose metabolized via the Embden-Meyerhof pathway. Reduced glutathione, on the other hand, after a small decrease associated with the "maturation" of reticulocytes into red cells, remains constant like the rate of the hexose monophosphate shunt.