A variety of estimators have been used by paleontologists for reconstructing body mass of fossil mammals. For rodents, the most commonly used proxy is m1 area, although that value is known to overestimate masses in muroid rodents and is generally problematic in rodent taxa in which the m1 is either enlarged (as in muroids) or reduced (as in extinct mylagaulids). This paper explores the potential utility of 2 alternative proxies, toothrow length and toothrow area; these measures are shown here to be very tightly correlated with body mass among rodents. Regression within certain clades of the Rodentia provides an even tighter fit. Applying these proxy measures to several fossil rodents provides an estimate of their masses. Comparison to published body masses of extinct rodents estimated from postcrania reveals that body masses for Castoroides and Palaeocastor estimated from femur length are consistent with those from dental proxies but that toothrow-based estimates for Phoberomys, the largest known fossil rodent, are discordant with the mass estimated from limb bone diameter. This difference may be explained by both allometric scaling of limbs and locomotor differences between Phoberomys and the caviomorph rodents on which the limb bone diameter regression was based.