A computer program has been used to calculate the effect of high shear stresses on viscosity measurement in capillaries. Viscous heating can occur long before the limiting Reynolds number is reached, and when it does, it will introduce serious errors in the viscosity calculation. The flow profile is flattened, causing an error to appear in the kinetic energy correction. The shear stress and shear rate at the wall are no longer constant, the shear stress decreasing and the shear rate increasing as the liquid flows down the capillary. As a result, the calculation of even an apparent viscosity gives a value that has questionable meaning. Heating effects can be reduced by using a shorter L/R ratio and a smaller radius.