Rates of mass specific oxygen consumption of two resting, postabsorptive howler monkeys at air temperatures of 24 to 26°C were 0.401 ml g−1h−1 (♂, 6.11 kg) and 0.480 ml g−1h−1 (♀, 3.23 kg). These data are about 5% in excess of the values predicted by Kleiber for mammals of similar body mass. Our data indicate that howler monkeys do not exhibit reduced standard metabolism as a consequence of being arboreal folivores. They appear to rely to a great extent on behavioral adaptations to help ameliorate energetic stresses potentially posed by diets high in foliage.