To compare the metabolic and circulatory responses to walking and jogging in water with similar exercise performed in air, oxygen uptake ([Vdot]O₂), ventilation ([Vdot]_E), respiratory exchange ratio (R), and heart rate (HR) were measured in six male subjects during 6-minute bouts of walking or jogging through waist-deep water (30 to 31°C) at five different speeds (.7–1 m/second). Work bouts of similar metabolic intensity were also carried out in air (27 to 29°C) on the treadmill. Approximately one-half to one-third the speed was required to work at the same level of energy expenditure during walking and jogging in water as compared with the exercise in air (2.6 to 3.5 vs. 5.5 to 13.4 km/ hour). During the water exercise, [Vdot]O₂, HR, and R increased nearly linearly with increased speed, whereas the increase in [Vdot]_E was somewhat curvilinear. These responses were similar to those during exercise in air. HRs at any given level of [Vdot]O₂ were not significantly different (p < .05) for the exercise in the two environments. It was concluded that during jogging in water, the effects of water resistance and buoyancy make possible high levels of energy expenditure with relatively little movement and strain on lower extremity joints, suggesting that this exercise may be a valuable alternate mode of conditioning for developing and maintaining work capacity and cardiovascular fitness. The similarity of the HR-[Vdot]O₂ relationship for the two environments indicates that the metabolic intensity of walking and jogging in warm water may be accurately prescribed from treadmill data by monitoring HR.