A quantitative model of the relaxation of transient lower ionospheric (D region) disturbances caused by lightning-induced electron precipitation is developed, taking advantage of known particular features of the lightning-induced disturbances, such as the fact that they are produced in typically <1 s and decay over 10--100 s. The model represents the nighttime D region as consisting of only four kinds of charged particles (electrons, positive ions, negative ions, and positive cluster ions) and is particularly suited for description of the detailed behavior of the electron density. Application of the model to some previously modeled disturbances indicates that some of the least known chemical reaction rates in the nighttime D region altitudes may be measurable using subionospheric VLF data. In the production of secondary ionization by precipitating electron bursts, the model calculations indicate the presence of a saturation effect such that the number density of the secondary electrons is not simply equal to the ion pair production rate times the burst duration. In some cases involving precipitation of ~1-MeV electrons, the model predicts the formation of new layers of ionization at 50--70 km altitude that represent a different attachment-detachment quasi-equilibrium value from that of the unperturbed ambient. Such new layers may exist for up to ~105 s following electron precipitation bursts. ¿ American Geophysical Union 1992