The magnitude and sign (positive) of the molecular hyperpolarizability &ggr; has been determined for several mixtures of nitrobenzene and benzene. This has allowed the testing of the adequacy of Onsager's local field factors, which are found to be valid. By comparing &ggr; of nitrobenzene with that of nitromethane, we find that the delocalized conjugated electrons are an order of magnitude more effective in contributing to &ggr; than are the localized nonconjugated electrons.