The oriented-gas approximation was examined at the molecular level by using an ab initio molecular orbital method in order to investigate its basic assumption : the additivity of hyperpolarizabilities between molecules. The additivity was examined in an interacting 2-methyl-4-nitroaniline (MNA) pair, the first-neighboring molecular pair in MNA crystal. The first-order hyperpolarizabilities beta of the MNA pair were estimated by using the supermolecule technique in coupled perturbed Hartee-Fock calculations, fully taking into account quantum mechanical intermolecular interactions between MNA molecules. The results indicated that hyperpolarizabilities were not additive between the molecules and the intermolecular interactions considerably influenced the hyperpolarizabilities of the MNA pair. The results were analyzed by using the electrical interaction model of Dykstra et al. (J. Mol. Struct. : THEOCHEM 1986, 113, 357), and it was found that electrostatic intermolecular interactions were the predominant influence on the hyperpolarizabilities, similar to the case of urea. The importance of the local field correction in the oriented-gas approximation was discussed from the molecular viewpoint, considering its microscopic basis which corrects the effects of electrostatic interactions between a molecule and the crystal field on hyperpolarizabilities within the framework of mean field theory.