Heavy exercise has been shown to elicit reductions in inspiratory muscle strength in healthy subjects. Our purpose was to determine the combined effects of moderate-intensity endurance exercise and a thermal load on inspiratory muscle strength in active subjects. Eight active, non heat-acclimatized female subjects (23.5 ± 1.4 yr; VO₂max = 39.8 ± 2.4 ml.kg^-1.min^-1) randomly performed two 40 min endurance exercise bouts (60% VO₂max) in either a thermo-neutral (22°C/21% RH) or hot (37°C/33% RH) environment on separate days. Maximal sustained inspiratory mouth pressure (PI_max) was obtained pre and post exercise as an index of inspiratory muscle strength. Additional variables obtained every 10 min during the endurance exercise bouts included: rectal temperature (T_RE), heart rate (HR), minute ventilation (V_E), oxygen uptake (VO₂), tidal volume (V_T), breathing frequency (F_b), and ratings of perceived exertion and dyspnea (RPE/RPD). Data were analyzed with repeated measures ANOVA. PI_max was significantly reduced (p < 0.05) after exercise in the hot environment when compared to baseline and when compared to post exercise values in the thermo-neutral environment. PI_max was unchanged from baseline following exercise in the thermo-neutral environment. HR and T_RE were significantly higher (p < 0.05) in the hot compared to the thermo-neutral environment. V_E and VO₂ were not significantly different between conditions. V_T was unchanged between conditions whereas F_b was higher (p < 0.05) in the hot condition compared to thermo-neutral. RPE was not significantly different between conditions. RPD was significantly higher (p < 0.05) in the hot compared to the thermo-neutral environment. We conclude that moderate-intensity endurance exercise (60% VO₂max) in a hot environment elicits significant reductions in inspiratory muscle strength in unfit females. This finding is novel in that previous studies conducted in a thermo-neutral environment have shown that an exercise intensity of >80% VO₂max is required to elicit reductions in inspiratory muscle strength. In addition, dyspnea perception during exercise is greater in a hot environment, compared to thermo-neutral, at a similar level of V_E and VO₂.

• The combined effects of a heat load and exercise on inspiratory muscle strength were investigated in untrained female subjects.

• Previous studies have shown that a very high exercise intensity (> 80% VOmax) is required to elicit reductions in inspiratory muscle strength.

• Prolonged submaximal exercise (40-min/60% VOmax) in a hot environment significantly reduced inspiratory muscle strength in untrained females whereas the same intensity in a thermo-neutral environment had no effect on inspiratory muscle function.

• These reductions in inspiratory muscle strength may be related to competition for blood flow among the locomotor, inspiratory, and cutaneous circulations.