Arctic ruminants consume cold water and food that increase costs of thermoregulation and potentially impair the bacterial fermentation on which these animals rely for digestion. We fed castrated adult muskoxen (Ovibos moschatus) a consistent diet throughout winter to correlate changes in ambient temperature with water flux, ingestive behavior, rumen temperature, and bacterial counts in the rumen. Ambient temperatures declined to −40°C but average ruminal temperatures were 39°C in muskoxen fed grass hay throughout winter. Rumen temperatures were not static but were punctuated by cold shocks to 26°C in each month. Water turnover rates were high in October (11.1 kg/day⁻¹) and low in January and March (9.8–7.7 kg/day⁻¹), which indicated a concomitant decline in food intake. Numbers of bacteria in rumen fluid decreased with water turnover and thus food intake from October (18.0 × 10⁹/ml) to January and March (9.4 and 8.8 × 10⁹/ml, respectively). The cost of warming ingesta was estimated at 25, 79, and 57 kJ/kg^0.75 in October, January, and March, respectively, and was 3–14% of the predicted intake of digestible energy. Muskoxen spent more time consuming water as snow than as free water, which may reduce the cold shock of water ingestion in winter. Concerted ingestion of food and water may also allow muskoxen to substitute heat increment of feeding for the cost of warming ingesta.