In acute in vivo exposure of rats to 25 to 250 ppm nitric oxide (NO) by use of a small exposure chamber for a single rat, the kinetic parameters of nitrosylhemoglobin (Hb-NO) and methemoglobin (MetHb) formation were estimated (with the aid of computer simulation) on the basis of experimental data. The biochemical and rheological injuries of erythrocytes were also examined. The time course of Hb-NO and MetHb formation in blood was compared with that simulated by a simplified kinetic model. The rate of MetHb formation from Hb-NO was much faster than MetHb reduction to ferrous form and dissociation of Hb-NO; thus, MetHb content was always greater than Hb-NO content. The activity of MetHb reduction decreased on exposure to a high concentration of NO, but the activity was recovered when rats were placed in clean air. Rheologically, the blood viscosity was scarcely altered, but a few undeformed cells were detected at high shear stress. Morphologically, echinocytic transformation was observed to some extent. Biochemically, the crosslinking of membrane proteins and the alteration of acyl chain composition of membrane phospholipids were not detected in the in vivo exposure, though the in vitro exposure of rat erythrocytes to high concentrations of NO revealed remarkable oxidative crosslinking among membrane proteins and hemoglobin. In conclusion, both for persistent methemoglobinemia and for membrane damage, the maintenance of reductive activity in erythrocytes is the most important determinant factor for the protection of NO-induced oxidative injury.