Rate of metabolism and body temperature were examined in 24 species of arvicolid rodents, for 15 of which data are presented here, to determine the factors that influence their level of energy expenditure. Arvicolids are characterized by a high, precisely-regulated body temperature, a high basal rate of metabolism by general mammalian standards, and a standard thermal conductance, except at large masses, when low conductances can occur. No evidence of entrance into torpor is known for any arvicolid; its absence is associated with especially high basal rates at masses smaller than 52 g. Arvicolids that live in cold climates, i.e. at high altitudes and latitudes, have higher basal rates than species that live in other environments. Basal rates, however, appear to be independent of food habits, presumably because of the small mass of most species and possibly because all species are herbivorous. A small size in combination with herbivory has permitted arvicolids to maintain continuous endothermy, which in turn has permitted them to exploit cool-to-cold environments by means of a high rate of production. The use of daily or seasonal torpor might well have prevented arvicolids from attaining the pivotal position in energy transfer that they presently occupy in many high-latitude communities.