By use of successive increments of discontinuous work with an arm-leg cycle ergometer the VO2, Q, SV, and HR were studied in six male subjects at rest and during exercise in air and in water at 18, 25, and 33 degrees C. The Q values obtained by CO2 rebreathing were reproducible. VO2 was linearly related to work with the plots for air and 33 degrees C water being similar. However, during work in 25 and 18 degrees C water, the VO2 averaged 9.0% (150 ml) and 25.3% (400 ml) higher, respectively, than values observed in 33 degrees C water, with the largest differences observed in leaner subjects. The plot of HR-VO2 was linear and almost identical during work in air and 33 degrees C water, but shifted significantly to the right in cooler water. VO2 averaged 250-700 ml higher in cold water compared to air and 33 degrees C water at a given mean heart rate. The Q vs. VO2 line was similar during work in air and in water with no effect of water or temperature. At similar levels of VO2, SV was significantly larger (P less than 0.05) in 25 and 18 degrees C water than in air or 33 degrees C water. Consequently, the reduction in heart rate during work in cold water was entirely compensated for by a proportionate increase in the SV of the heart. Q was therefore maintained at similar levels of energy expenditure in air and in 18, 25, and 30 degrees C water.