Ten subjects performed standard exercise tests at two submaximal loads and one maximal load before and 90 min after dehydration caused predominantly by 1) a thermal, 2) a metabolic, and 3) a combined thermal and metabolic heat load applied for 2.5@#X2013;4 hr. Each subject interrupted dehydration so that almost the same decrease in body weight was attained in the three situations (1.7@#X2013;4.6 kg). Oxygen uptake, heart rate, and concentration of blood lactate were measured during the exercise. At the submaximal loads there was no change in oxygen uptake after dehydration but the heart rates were significantly higher (mean difference 13 beats/min) and blood lactates were lower (from 0.5 after (1) to 1.6 (2) mmoles/liter). At the maximal load there were no significant changes in oxygen uptake and heart rate but work times decreased markedly (6@#X2013;4 min) as did blood lactates (14.0@#X2013;10.4 mmoles/liter) especially after exercise dehydration. physical work capacity Submitted on January 20, 1964