Vitamin E is a fat-soluble antioxidant which plays an important role in maintaining cells in a reduced state. Oxidation reactions can lead to damage of both endothelial cells and circulating blood cells and may thus influence the rheological conditions. A group of 13 mountaineers was selected as a model for persons at increased risk of oxidative stress. 6 subjects received 200 mg vitamin E twice daily for 4 weeks, and 7 subjects received placebo. Erythrocyte filterability, blood viscosity, changes in the blood picture, and three blood coagulation factors (antithrombin III, protein C, and fibrin monomers) were investigated. The baseline values (t1) were determined at 1.500 m, and after supplementation the investigations were repeated twice at 4.300 m (t2 und t3). There was a marked rise in the hematocrit in both groups during the ascent which was due to an increase not only of the erythrocytes but also of the leucocytes. This change was more pronounced in the control group. The erythrocyte filterability was unaltered in the vitamin E group in comparison with baseline but was significantly impaired in the control group. The changes in these two parameters-hematocrit and filterability--resulted in a significant higher blood viscosity. Furthermore in the control group, but not in the vitamin E group, a significant fall in the protein C activity was observed. The cause may be an additional release from degenerated leucocytes of various proteases which degrade protein C. A further possible cause is a derangement of metabolic reactions in the vascular endothelium. All these possible causes could be counteracted by the higher antioxidative potential of the verum group.(ABSTRACT TRUNCATED AT 250 WORDS)