Aerial oxygen consumption of unrestrained, freely-diving warm- and cold-acclimated snapping turtles, Chelydra serpentina, was measured at 10, 20, and 30 C. Also, simultaneous determinations of aerial and aquatic oxygen uptake by voluntarily-diving animals were made at 4 and 20 C. The standard rates of aerial oxygen consumption are equivalent in cold- and warm-acclimated animals in water and in cold-acclimated ones in air; these rates are all lower than those of warm-acclimated animals in air. Thus either cold acclimation or voluntary submergence reduces the standard metabolic rate of snapping turtles but the effects are not additive. Aquatic oxygen uptake during voluntary submergence is more important at low than at moderate temperatures and probably contributes significantly to gas exchange in these animals as they overwinter beneath the ice of ponds and streams.