Three relatively fit subjects performed duplicate 20- to 25-min cycle ergometer exercise bouts at moderate and heavy intensities (40% and 70% Vo2 max) in ambient temperatures of 20, 26, and 36 degrees C. They approached a steady state in internal body temperature (Tes) in all but the heavy exercise in the heat, where Tes rose consistently, averaging 38.84 degrees C at the termination of exercise. Cardiac output (Q), estimated by a rebreathing technique, was proportional to Vo2 and independent of the body temperatures, except during the lower exercise intensity in the heart, where Q averaged 1.31 . min -1 higher throughout. In any environment, forearm blood flow was linearly related to Tes above the Tes threshold for vasodilation, but during heavy exercise in the heat this relationship was severely attenuated above a Tes around 38.0 degrees C, when forearm blood flow exceeded 15 ml.min -1 .100 ml -1. Plasma volume decreases during exercise were primarily a function of the intensity of exercise. During heavy exercise in the heat the relative vasconstriction contributes to the maintenance of an adequate stroke volume preventing a fall in Q. In this case, circulatory regulation has precedence over temperature regulation.