The aim of this paper is to form a derived (incremental) type of constitutive law for non-isothermal elasto-plastic deformation of metals. This was achieved by extending the analysis performed in [2 - 5] for the isothermal case. The obtained result, i.e. constitutive laws (4.15) and (4.16), appear derived for the first time in this paper. They are not limited to the case of small deformations and their only major limitations stem from the assumption of isotropic deformation and the absence of time-dependent (viscous, rheological) effects.