Among 17 species of mammals, field metabolic rates exclusive of thermoregulatory and productive costs (designated FMR*) averaged 2.65 × standard metabolism (SMR). Daily activity costswere calculated by subtraction from FMR* of the daily energy expenditure associated with SMR and assimilation of nutrients. Total expenditure for activity was of a similar magnitude to that for daily standard metabolism. Calculations indicate that expenditures by mammals for locomotion probably account for less than half of daily activity costs. Expenditures by mammals engaged in other kinds of activities are also reviewed. During their daily activity periods, terrestrial mammals expend energy at a rate of about 4.1 × SMR. The utility of energetic increments for activity in time-energy budgets, thermal energy budgets, and analyses of the economics of foraging are discussed.