Hyperglycemia is likely to be one of the important determinants of ion transport as it is known to induce oxidative stress and may thus enhance non-specific permeability of membranes. The aim of the present study was to evaluate the effects of an acute increase in glycemia on Rb-86(+) (a marker for K+) influx and lipid peroxidation. We evaluated the 75-g oral glucose tolerance test (OGTT)-induced modification on Rb-86(+) influx and plasma lipid peroxidation in 20 subjects with normal glucose tolerance (NGT). After 2-hour glucose loading, the levels of passive Rb-86(+) influx and plasma lipid peroxidation were significantly increased, whereas the active influx of Rb-86(+) was unchanged. The total and passive influx of Rb-86(+) into erythrocytes was significantly correlated with the level of plasma lipid peroxidation. This study demonstrates that acute hyperglycemia induces an increase in the passive influx of Rb-86(+) in subjects with NGT, suggesting that acute hyperglycemia may produce an oxidative stress in plasma. These changes may be among the earliest changes occurring in response to hyperglycemia. ((C))2001, Editrice Kurtis.