Explicit use of the poloidal Ohm's law, together with the radial plasma velocity (obtained from the distribution of plasma sources), leads to a very general constraint on the possible radial profiles of plasma density and temperature. The constraint does not require any ad hoc assumption; it can place severe restrictions on the allowed profiles and is independent of energy and particle transport; also, it may be the underlying principle of profile consistency. The constraint is discussed in the framework of neoclassical theory, using results from the Frascati tokamak.