The rate of transport of [32P] phosphate into human and bovine erythrocytes and of a spin-label analogue of phosphate (Tempo-phosphate) into human erythrocytes was found to decrease with increasing erythrocyte age by 15-20% when comparing 20% most dense cells with 20% of lightest cells. The activation energy of Tempo-phosphate transport did not show significant changes upon erythrocyte aging.