Since the dehydration causes a loss of body water, we studied the rheological properties of blood in the course of water deprivation. Subjects used in this study were 64 white male rats divided into 4 groups: control (n=19) and 3 experimental groups which underwent water deprivation for 3 days (n=15), 6 days (n=15) and 10 days (n=15). The results obtained indicate that under dehydration animals have higher blood and plasma viscosity and erythrocyte aggregation index than in the control group. After 3 days of dehydration these changes are due to the loss of intravascular water. The water deprivation for 10 days causes significant disturbances in blood composition as well as changes of red blood cell membrane properties whereas blood and plasma volume return to control values.