Components of the crystal quadratic susceptibility tensor x(2) for second-harmonic generation are calculated for the title compounds 2-(α-methyl benzylamino)-5-nitropyridine (MBANP) and 4-nitro-4'-methyl(benzylidene aniline) (NMBA). Input data are the crystal structure and refractive indices and CNDO hyperpolarizabilities. The calculations also yield effective polarisabilities and local electric fields. Susceptibility components reach 66 pm V−1 in MBANP and 27 pm V−1 in NMBA in the crystal axes; agreement with experiment is poor for MBANP but satisfactory for NMBA. These features seem to reflect the better defined charge transfer axis in NMBA. Screened dipole–dipole interactions calculated from CNDO dipole moments stabilise MBANP by 70 kJ mol−1 and NMBA by 20 kJ mol−1 and imply permanent electric fields of a few GV m−1.