The purpose was to investigate the mechanism for the excessive exercise hyperthermia following deconditioning (reduction of physical fitness). Rectal (T _re) and mean skin ( [`(T)]<sub>sk</sub>\bar T_{sk} ) temperatures and thermoregulatory responses were measured in six men [mean (SD) age, 32 (6) years; mass, 78.26 (5.80) kg; surface area, 1.95 (0.11)m<sup>2</sup>; maximum oxygen uptake ( [(V)\dot]O<sub>2max</sub>\dot VO_{2max} ), 48 (6) ml·min<sup>–1</sup>·kg<sup>–1</sup>; whilst supine in air at dry bulb temperature 23.2 (0.6)°C, relative humidity 31.1 (11.1)% and air speed 5.6 (0.1) m·min<sup>–1</sup>] during 70 min of leg cycle exercise [51 (4)% [(V)\dot]O<sub>2max</sub>\dot VO_{2max} ] in ambulatory control (AC), or following 6 h of chair rest (CR), 6° head-down bed rest (BR), and 20° (WI20) and 80° (WI80) foot-down water immersion [water temperature, 35.0 (0.1)°C]. Compared with the AC exercise [`(T)]_sk\bar T_{sk} responded similarly to exercise: they decreased (NS) by 0.5–0.7°C in minutes 4–8 and equilibrated at +0.1 to +0.5°C at 60–70. Skin heat conductance was not different among the five conditions (range = 147–159 kJ·m^–2·h^–1·°C^–1). Results from an intercorrelation matrix suggested that total body sweat rate was more closely related toT _re at 70 min (T _re70) than limb sweat rate or blood flow. Only 36% of the variability inT _re70 could be accounted for by total sweating, and less than 10% from total body dehydration. It would appear that multiple factors are involved which may include change in sensitivity of thermo- and osmoreceptors.