A method for studying blood viscosity at low shear rates (from 4 x 10(-2) to 7 x 10(1) sec-1), i.e. in the shear range in which higher is the influence of shear rate on viscosity, was developed. Blood viscosity is expressed by the two regression coefficients of a power regression, according to the function: eta = a . Db where eta is viscosity. D is the shear rate, a and b are the two regression coefficients, which are independent of the shear rate. We found that the regression power was highly significant in a wide hematocrit range. Practically, this work deals with the possibility of application to the blood of the Ostwald formula for pseudoplastic flow behavior. We found significant correlations between the two regression coefficients and hematocrit and plasma viscosity, so correcting by calculation the two regression coefficients for standard hematocrit and standard plasma viscosity. This method is simple and can easily be automatically driven by a computer.