Acute exposure to high altitude for 1 day in rats caused an increase in hematocrit (Hct) with no change in mean arterial blood pressure (mABP) from the non-altitude control, whereas after prolonged exposure to altitude (5-6 weeks) there were increases in both Hct and mABP. No changes in total plasma protein (TPP) and plasma osmolality (POsm) from control rats were observed in all altitude-exposed animals. The ability of the acutely and chronically altitude and non-altitude exposed rats to resist hemorrhage was studied. Hemorrhage was standardized at mABP in the range of 30-35 mmHg. Chronic exposure to altitude increased the initial and maximum volume of blood withdrawn as well as the oligemic and survival times, whereas acute altitude exposure did not. The higher ability to resist standardized hemorrhagic shock of the chronically altitude exposed rats seemed to result, in part, from their greater hemodilution and better arterial blood pressure regulation. No difference in the rate of hemodilution as well as hemorrhagic tolerance was observed between the 1-day altitude and control rats. The difference in rate of hemodilution between the chronic altitude and control animals could not be due to arterial hyperosmolality since the magnitude of change in POsm during blood loss was the same for all animals groups.