In a type-II superconductor the pinning properties and, in particular, the current dependence of the thermal activation energy U(J) determine the shape of the current–voltage (I–V) curve. The generation of higher harmonics in the magnetic response to an alternating magnetic field is a consequence of the non-linearity in the I–V characteristic. In this work we report our analysis of fundamental (χ1) and third (χ3) harmonics of the ac magnetic susceptibility on MgB2 bulk superconductors. In particular, measurements have been performed as a function of the temperature and the dc magnetic field, up to 9 T, for various frequencies and amplitudes of the ac magnetic field. By comparing the experimental susceptibility curves with numerical simulations of the magnetic diffusion in a type-II superconductor, we show that the frequency dependence of the third harmonic response can provide clear information about the characteristics of the non-linearity in the I–V curve and, therefore, the dynamical regimes governing the vortex motion in the analyzed sample