Convenient methods were developed for measuring maximum oxygen consumption (VO2 max) in untrained small mammals during treadmill exercise and cold exposure. Deer mice (Peromyscus maniculatus) were run once, for 6-min periods at velocities exceeding maximal aerobic running speed, while instantaneous oxygen consumption was measured. The VO2 max during cold exposure was determined using high wind speeds to increase heat loss rates. During running, the kinetics of gas exchange were rapid and similar to those observed in larger mammals. Half-times of the VO2 and VCO2 on-responses were about 20 sec. The VO2 also increased rapidly during suddenly imposed cold stress, reaching VO2 max within 1-1.5 min. Values of VO2 max obtained during exercise were similar to the VO2 max measured during cold exposure.