Normal human erythrocytes suspended in isotonic saline at 0.5 haematocrit displayed, after 30 min exposure to 1 mM tert-butylhydroperoxide at 37 degrees C, a marked increase of NADPH, while the concentration of the other adenine nucleotides was almost unchanged. Hereditary Spherocytosis and glucose-6-phosphate dehydrogenase-deficient red blood cells exhibited, under basal conditions, higher levels of most of the nucleotides assayed and significant amounts of hypoxanthine. Treatment with tert-butylhydroperoxide caused, in glucose-6-phosphate dehydrogenase-deficient erythrocytes, a pronounced decrease of ADP and of AMP levels, a substantial invariance of other adenine nucleotides and a considerable raise of hypoxanthine. On the contrary, Hereditary Spherocytosis erythrocytes exhibited, after oxidative stress, increased levels of ADP and of AMP, a slight decrease of ATP and an accumulation of hypoxanthine similar to that found in enzyme-deficient red cells. In both the pathologic erythrocytes the addition of phosphate during the oxidative treatment resulted in a lower formation of hypoxanthine, while the presence of 10 mM glucose, fully prevented its appearance.