Shrubs are increasing in the annual range of arctic caribou (Rangifer tarandus (L., 1758)), but it is unknown how much summer browse caribou could consume. We measured instantaneous intakes of resin birch (Betula glandulosa Michx.) and feltleaf willow (Salix alaxensis (Andersson) Coville) by caribou during summer. Daily intake of a formulated diet without toxins was measured during the same period to monitor appetite. Caribou appetite increased from 64.1 to 86.7 g DMkg0.75day1 as animals gained body mass from 96.8 to 113.5 kg. We estimated that caribou required 645 kJkg0.75day1 of digestible energy to maintain body mass and 1113 kJkg0.75day1 to gain body mass for autumn reproduction. Caribou had the same bite mass (9.7 mgbite1kg0.75) and instantaneous intake rate (0.17 g DMmin1kg0.75) on both forages; however, birch contained more phenols (3.3% vs. 1.5%) and less available protein (6.2% vs. 10.2%) than willow. A 100 kg female caribou would need to consume 2.48.7 kg of fresh browse, requiring 3.18.5 hday1 of eating time to meet daily energy requirements. Birch is unlikely to provide enough nitrogen for maintenance of body protein. Therefore, caribou may depend on abundance and diversity of plants to offset toxin loads and low protein intake from shrubs during summer.