The aim was to define hemorheological changes (and their kinetics) in a non-exercising group of subjects after exposure to moderately high altitude of 2700m. 12 men were transported to this height without marked physical exercise and remained there for 6 days. During this time there was a significant increase in blood and plasma viscosity as well as fibrinogen. Hematocrit, blood cell counts and ESR did not change significantly. All alterations were reversible upon return to normal height. These findings suggest that a viscidation of blood occurs as a result of exposure even to moderate height.