Exercise in an aquatic environment may be an effective mode of therapy andtraining due to reduced impact forces. The purpose of this study was to comparethe physiological responses of walking/running on a land treadmill withwater treadmill responses at two different depths. Six subjects completedwalking and running trials on both a land-based and a water-based treadmill.Water-based trials were completed in both thigh- and waist-deep water.Each trial was five minutes in duration. Oxygen uptake (VO2), heart rate (HR),respiratory exchange ratio (RER), stride frequency (SF), and the oxygen costper stride (VO2/stride) were compared between the conditions using a twowayANOVA with repeated measures. Walking and running in water elevatedVO2 (p < 0.02) and HR (p < 0.04) above land treadmill values. When runningin waist-deep water, VO2 and HR failed to increase to the same extent asthigh-deep running. Stride frequency was similar between the three differentdepths during walking but lower in waist-deep water during running. VO2/stride was significantly higher (p < 0.01) in water-based walking and runningcompared to land-based values. Water-based walking and running eliciteda greater physiological cost than land-based exercise, which can be attributedto the elevated cost of moving in water due to increased resistance.When running in waist-deep water, buoyancy may counter the resistance ofthe water and serve to lower the physiological cost of locomotion.