The binding of arsenite, As(III), and arsenate, As(V), by molecules in the intracellular compartment of rabbit erythrocytes has been studied by 1H- and 31P-NMR spectroscopy, uptake of 73As, and ultrafiltration experiments. For intact erythrocytes to which 0.1-0.4 mM arsenite was added, direct evidence was obtained for entry of 76% within 1/2 h and subsequent binding of As(III) by intracellular glutathione and induced changes in the hemoglobin structure (NMR), likely due to binding of As(III). These results were compared with the effect of addition of As(V) on intact erythrocytes and revealed that a smaller amount of As(V) (approximately 25%) enters the cells; the main fraction of As(V) enters the phosphate pathway, depletes ATP, and increases Pi. In contrast, As(III) did not affect the ATP level. Both 1H- and 31P-NMR data indicated striking differences between As(III) and As(V) behavior when incubated with rabbit erythrocytes. These differences were confirmed by 73As uptake and binding experiments. meso-2,3-Dimercaptosuccinic acid (DMSA), a dithiol ligand, released glutathione from its arsenite complexes in erythrocytes.