The molecular reorientational dynamics of the side-chain nonlinear optical (NLO) polymer [poly(methyl methacrylate-co-p-nitrophenylprolinol)] were investigated. A nonlinear optical response, second-harmonic generation (SHG), was used to probe the induction and decay of molecular orientational order. A two-mode growth of the SHG signal induced by the poling field at temperatures above T(g) and a subsequent two-mode SHG relaxation in the absence of the poling field at temperatures below T(g) were observed. A qualitative interpretation that distinguished between the motion of the side-chain NLO chromophores decoupled from polymer segmental movement and the motion of the chromophores coupled to the polymer segmental movement was proposed to explain the observed two-mode poling and relaxation phenomena. Under the influence of an electric poling field, after a rapid response of the side-chain chromophore, the polymer chain contour rearranges itself and creates an anisotropic local environment. This field-induced anisotropic local environment in turn enhances the orientational ordering of the NLO chromophores and also significantly influences the relaxation behavior after the poling field is removed. A poling field-jump experiment gave direct evidence that the polymer backbone undergoes anisotropic rearrangement during the poling process. The significant influence of the field-induced polymer backbone rearrangement of the SHG relaxation behavior was manifested in a correlation experiment contrasting the poling process and the SHG relaxation behavior.