Growth rates and/ or dynamics of body mass for the hispid cotton rat (Sigmodon hispidus), fulvous harvest mouse (Reithrodontomys fulvescens), and marsh rice rat (Oryzomys palustris) were determined from field data. Significant sexual dimorphism was exhibited by all three species when based on average body mass. Growth rates were categorized by sex, season, and size class for Sigmodon and Reithrodontomys. Female Sigmodon grew faster than males, there were no seasonal differences in growth rate for cotton rats, and Reithrodontomys exhibited increased spring female growth and overall decreased growth rate with increased size. The lack of seasonal differences in growth rates was attributed to the mild climate of the area and the continuous availability of resources. Growth rates determined in the field were, on the whole, much lower than growth rates obtained from laboratory colonies.