In a recent series of articles Gruntfest has obtained numerical solutions of the equations relating the effects of a dissipative process like viscous flow on the observable flow patterns of a heat‐sensitive material. Gruntfest has implied that non‐Newtonian flow is at least partially due to this thermal feedback, and he has demonstrated its significance in the flow of polymer‐modified oils reported on by Philippoff. This raises the question of how important these effects are in general, i.e., is non‐Newtonian flow an experimental artifact. In this paper the qualitative and quantitative predictions of Grantfest's analysis are examined. These predictions are then compared to some experimental data obtained with simple fluids, polymer solutions, polymer melts, and colloidal dispersions. It is shown that non‐Newtonian flow usually appears well before the rates of shear at which thermal effects would be noticed. Hence, non‐Newtonian flow is not an experimental artifact brought about by the inability to transfer thermal energy away from heat‐sensitive materials.