Methemoglobin levels have been found to vary with altitude and to shift the hemoglobin-oxygen dissociation curve. In this study, hematocrits and methemoglobin levels were monitored in rats exposed to hypoxia (420 torr absolute) for various intervals. Hematocrits gradually increased throughout the period of hypoxia, while methemoglobin levels rose by 12 h, peaked at 24 h and returned to control level by day 6. These data, in the context of other work, suggest that increased methemoglobin is important in acclimation to hypoxia.