AIMS: Cardiac autonomic modulation, as assessed by heart rate variability (HRV), is independently attenuated by both type 1 diabetes (T1D) and exercise-heat stress, although their combined effects remain unclear. We therefore assessed HRV during exercise-heat stress in young individuals (18-37 years) with (n = 14) and without type 1 diabetes (n = 14). METHODS: Participants completed 30-min seated rest and three, 30-min bouts of semi-recumbent cycling at light, moderate, and vigorous metabolic heat productions (200, 250, 300 W/m(2), respectively), each followed by 30-min recovery. Body core temperature (T(core)) and electrocardiogram were recorded throughout and analyzed during the final 5-min of rest and each exercise period. RESULTS: Relative to baseline, T(core) was increased in both groups, albeit to a greater extent in type 1 diabetes during vigorous exercise (T1D, 1.1 +/- 0.3 degrees C; control, 0.8 +/- 0.3 degrees C; P < 0.05). Overall HRV (as reflected by entropy) was attenuated throughout exercise relative to baseline in both groups, with the magnitude of the reduction greater in type 1 diabetes during vigorous exercise (T1D, - 108%; control, - 70%; P < 0.05). CONCLUSIONS: Given the negative correlations between decreased HRV and cardiac risk, our novel observations indicate that vigorous exercise in hot environments may pose a health concern for individuals with type 1 diabetes.