Rates of CO2 production in free—living and captive howler monkeys were measured using doubly labeled water, and assimilation of energy from natural foods was determined. Results permit construction of a field energy budget, and estimation of the feeding rate of these arboreal, plant—eating primates. Field metabolic rates averaged 355 kJ kg—1 day—1 (°2 x basal metabolic rate). Assimilation of the energy in a fruit and leaf diet was °40%. Feeding rate in the field is estimated to be °54 g dry matter kg— day—1, or °90 kg dry matter ha—1 yr—1.