It was demonstrated that the appearance of sickle-shaped erythrocytes can be induced by oxygenating the blood of European red deer (Cervus elaphus). The process was reversible. Blood cell counts with the exception of a slight reduction in red cell number and protein fractions did not show any abnormal values. The blood, which was examined at least 18 h after sampling, exhibited considerable deviations of blood gas values. In addition, highly significant haemorheological changes due to the sickle-shaped and oxygenated erythrocytes were detected.