We conducted a mark-recapture experiment to examine population dynamics of the fulvous harvest mouse (Reithrodontomys fulvescens) and plains harvest mouse (R. montanus) in response to low-level nitrogen amendments (16.4 kg N/ha/y) in an old-field grassland. The experimental design consisted of 16, 0.16-ha plots with four replicates of each treatment combination (fenced, nitrogen amendment; unfenced, nitrogen amendment; fenced, control; unfenced, control). We predicted that densities, survival, and transition probabilities would be greater for both species on nitrogen-amended plots because of greater aboveground biomass (i.e., enhanced concealment from predators). We observed no distinct patterns in survival or transition probabilities of R. fulvescens or R. montanus with regard to treatments. Although population densities of R. fulvescens did not exhibit any distinct patterns with regard to treatments, densities of R. montanus tended to be highest on nitrogen plots, but lowest on nitrogen-fenced plots during winter 1999–2000. As low-level nitrogen amendments continue to be applied, we predict survival and densities of R. montanus and R. fulvescens on control plots, especially fenced plots with no nitrogen amendment, will eventually exceed those on nitrogen-amended plots as a result of higher plant species diversity, food availability and better quality cover.