Context: Quanti!cation of the magnitudes of "uid resistance provided by water jets (currents) and their effect on energy expenditure during aquatic-treadmill walking is lacking in the scienti!c literature. Objective: To quantify the effect of water-jet intensity on jet velocity, drag force, and oxygen uptake (VO2) during aquatictreadmill walking. Design: Descriptive and repeated measures. Setting: Athletic training facility. Participants, Intervention, and Measures: Water-jet velocities were measured using an electromagnetic "ow meter at 9 different jet intensities (0–80% maximum). Drag forces on 3 healthy subjects with a range of frontal areas (600, 880, and 1250 cm2) were measured at each jet intensity with a force transducer and line attached to the subject, who was suspended in water. Five healthy participants (age 37.2 ± 11.3 y, weight 611 ± 96 N) subsequently walked (~1.03 m/s or 2.3 miles/h) on an aquatic treadmill at the 9 different jet intensities while expired gases were collected to estimate VO2. Results: For the range of jet intensities, water-jet velocities and drag forces were 0–1.2 m/s and 0–47 N, respectively. VO2 increased nonlinearly, with values ranging from 11.4 ± 1.0 to 22.2 ± 3.8 mL · kg–1 · min–1 for 0–80% of jet maximum, respectively. Conclusions: This study presented methodology for quantifying water-jet "ow velocities and drag forces in an aquatic-treadmill environment and examined how different jet intensities in"uenced VO2 during walking. Quanti!cation of these variables provides a fundamental understanding of aquatic-jet use and its effect on VO2. In practice, the results indicate that VO2 may be substantially increased on an aquatic treadmill while maintaining a relatively slow walking speed.