A two-dimensional mode propagation model has been used to investigate the effect of localized ionospheric perturbations on the propagation of VLF radio waves in the earth-ionosphere waveguide. Computations have been performed for the NSS transmissions at 22.3 kHz from Annapolis, Maryland to Eights Station, Antarctica, where anomalous short-duration signal amplitude changes, in coincidence with whistlers, were first noted. The calculations suggest that two ionization regions, one very near the transmitter and another near the receiver, may need to be present at the same time in order to obtain the large amplitude increases (>3 dB) that are observed to occur on occasion. It is suggested that the former is due to transmitter-induced electron precipitation, while the latter is due to precipitation associated with the whistler. We also find that small amplitude changes can be obtained under less restrictive ionospheric perturbation conditions.