In 1982 and 1983. the population demography of M. pennsylvanicus was not altered significantly by single or double applications of Orthene. Although inconsistencies in population size, survival, and recruitment were not explained by differences between control and experimental grids in breeding, emigration, interspecific competition, or pesticide-induced mortality, there may have been fundamental microhabitat differences between grids in both years, and this is supported by prespray data. Slight differences between grids may also reflect the random error associated with the models used to estimate parameters. No evidence was found to indicate an effect on relative weight change or average distances moved. The level of brain AChE in 1982 during sequential days after spraying was significantly lower than control levels. However, this inhibition was not enough to cause direct mortality. Recovery of brain AChe was gradual, probably reflecting continual re-exposure through the diet. Acephate and methamidophos residues were present in the vegetation collected from sprayed areas immediately following treatment, but were reduced or absent after 8 days. AChE inhibition in 1982 and 1983 extended beyond the disappearance of residues in the vegetation. Residues were only present in the G.l. tracts of voles collected immediately following treatment due to rapid metabolism and excretion. Levels in the vegetation and G.l. tracts in 1982 and 1983 were well below the rat oral LD50 of acephate. It is concluded that Orthene applied at recommended levels, in a single or double treatment, should not affect unconfined populations of M. pennsylvanicus in Maryland old-field habitats. Furthermore, this study confirms this method as a sensitive means to determine the overall impact of insecticides on wildlife populations. However, caution must be exercised to insure that all control and experimental areas are closely scrutinized for habitat differences.