The relationship between erythrocyte nucleotide profiles and the presence of atherosclerotic peripheral occlusive vascular disease was investigated. In elderly male patients with severe vascular insufficiency the mean red cell content of NAD, GDP, GTP, AMP, ADP and ATP and the cellular adenylate energy charge were not significantly different from those observed in young healthy males. It has been claimed that drugs such as oxpentifylline improve peripheral tissue oxygenation in vascular disease by reversing the fall in red cell ATP content which has been reported to accompany vascular insufficiency resulting in a restoration of normal cell deformability and hence whole blood viscosity. We have carried out in vitro studies using erythrocytes from normal adults to assess the effect of oxpentifylline on erythrocyte ATP content and glycolytic rate. The drug failed to significantly affect the rates of glucose consumption and lactate production or the ATP content of the erythrocytes compared with controls. Furthermore the drug did not influence the rate of ATP utilisation by erythrocytes. We conclude that red cell ATP and total adenylate content is not different from normal in patients with peripheral vascular disease. If oxpentifylline alters red cell deformability it does so by some mechanism not related to the cellular ATP concentration, the cellular adenylate energy charge or the glycolytic rate.