Little is known about abundance–climate relationships for animals because few long-term field studies address the issue. We present results from a 20-year study of abundance of Peromyscus leucopus (white-footed mouse) and Microtus pennsylvanicus (meadow vole). P. leucopus exhibited a biennial cycle with an overall population increase. M. pennsylvanicus did not exhibit a population cycle, and population size declined. We attribute long-term changes in abundance of both species to succession from grass-dominated to tree-dominated vegetation. The only significant association we found between abundance of either species over 20 years and 4 climate variables was a positive relationship between M. pennsylvanicus and summer precipitation. However, all but 1 of the potential abundance–climate relationships were significant when temporal subsets of the original data were analyzed. Weak associations may exist between climate and abundance that are contingent upon other unidentified factors, which further suggests that abundance–climate correlations are sensitive to the particular time frame of a study.