Abstract <p>The results of the rheological study of aqueous solutions of poly(<i>N</i>-vinylpyrrolidone) in the shear flow mode at different concentrations and temperatures in a magnetic field and in its absence are presented. The influence of a magnetic field on the enthalpy of activation of a viscous flow process Δ<i>H</i>_visc was studied for the first time; it value was calculated according to the Frenkel–Eyring equation based on data on the temperature dependence of the viscosity of solutions. It was found that in a magnetic field the value of Δ<i>H</i>_visc is higher than that outside the field. This is due to the additional orientation of diamagnetic macromolecules and the strengthening of interchain interaction, which promotes the association of macromolecules. This is confirmed by the increase in viscosity. A nonmonotonic increase in Δ<i>H</i>_visc is observed with increasing concentration of poly(<i>N</i>-vinylpyrrolidone) in solutions.</p>