Summary.It is confirmed that ATP levels are significantly increased in uraemic (1.90±0.06 mmole/l RBC) as compared to control RBCs (1.30 ± 0.03 mmole/l RBC) (P0.001). Studies were performed which suggest that this increase in ATP may be secondary to an increased ATP synthetic capacity in uraemic erythrocytes which is independent of extracorpuscular factors including phosphate: (I) When suspended in an artificial low phosphate (PO4) medium (I mM), washed RBCs from uraemic subjects produced and utilized ATP more rapidly than did washed control RBCs. (2) During a 12 hr incubation period, the ATP concentration of washed uraemic RBCs suspended in a 10 mM PO2medium remained significantly greater than the ATP level of washed control RBCs suspended in a 10 mM PO4medium. (3) During a 6 hr period of haemodialysis when the serum PO4level fell sharply, the RBC‐ATP failed to change. Pyruvic kinase activities were significantly increased in uraemic (7.02 ± 0.84 EU/g Hb) as compared to control RBCs (3.20 ± 0.17 EU/g Hb) (P0.001), suggesting the presence of young RBCs in uraemia. It is proposed that the increased ATP levels and intrinsic ATP synthetic capacity may be due to the presence of an erythrocyte population with a younger mean age in the uraemic population st
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