Hemorheological activity of 4-methyl-2,6-di-isobornyl phenol, a new o-isobornyl phenol derivative, was studied under conditions of experimental prolonged partial cerebral ischemia. Brain ischemia is associated with hemorheological disorders which can be characterized as blood hyperviscosity syndrome: increased viscosity of the whole blood (within a wide range of shear rates), plasma viscosity, fibrinogen content in blood plasma, and platelet aggregation; deterioration of platelet deformability and reduced availability of oxygen for tissues. A course (5 days) of intragastric 4-methyl-2,6-di-isobornyl phenol (100 mg/kg) prevented the development of blood hyperviscosity syndrome by modulating blood macrorheology (reduction of plasma viscosity and fibrinogen content) and microrheology (reduction of erythrocyte aggregation and improvement of their deformability).