Increased blood viscosity facilitates the formation of thrombosis. This is an important risk factor in the occurrence of cerebral infarctions. The present study was undertaken to elucidate whether docosahexaenoic acid (DHA) inhibits blood viscosity, hematocrit and fibrinogen in the disease animal model, stroke-prone spontaneously hypertensive rats (SHRSP). An attempt was also made to clarify the effect of DHA on blood pressure in SHRSP. Blood viscosity, hematocrit and fibrinogen in non-treated SHRSP increased significantly when compared with levels in age-matched non-treated Wistar Kyoto rats (WKY). SHRSP rats which were administered DHA for 5 weeks displayed significant decreases in blood viscosity, hematocrit and fibrinogen when compared with the values in non-treated SHRSP. The blood pressure of DHA-treated SHRSP was significantly lower than that of non-treated SHRSP. A positive correlation existed between blood pressure and blood viscosity. These findings suggest that decreased blood viscosity induced by DHA appears to be associated with the reduction of thrombosis formation and hypotensive action in SHRSP.