BackgroundHeat stress during aerobic exercise training may offer an additional stimulus to improve cardiovascular function and performance in a cool-temperate environment. However, there is a paucity of information on the additive effects of high-intensity interval exercise (HIIE) and acute heat stress. We aimed to determine the effects of HIIE in combination with acute heat stress on cardiovascular function and exercise performance.MethodsTwelve active (peak O2consumption [VO2peak]: 47 ± 8 ml·O2/min/kg) young adults were counterbalanced to six sessions of HIIE in hot (HIIE-H, 30 ± 1 °C, 50 ± 5% relative humidity [RH]) or temperate conditions (HIIE-T, 20 ± 2 °C, 15 ± 10% RH). Resting heart rate (HR), HR variability (HRV), central (cBP) and peripheral blood pressure (pBP), peripheral mean arterial pressure (pMAP), pulse wave velocity (PWV), VO2peak, and 5-km treadmill time-trial were measured pre- and post-training.ResultsResting HR and HRV were not significantly different between groups. However, expressed as percent change from baseline, cSBP (HIIE-T: + 0.9 ± 3.6 and HIIE-H: -6.6 ± 3.0%,p= 0.03) and pSBP (HIIE-T: -2.0 ± 4.6 and HIIE-H: -8.4 ± 4.7%,p= 0.04) were lower in the heat group. Post-training PWV was also significantly lower in the heat group (HIIE-T: + 0.4% and HIIE-H: -6.3%,p= 0.03). Time-trial performance improved with training when data from both groups were pooled, and estimated VO2peakwas not significantly different between groups (HIIE-T: 0.7% and HIIE-H: 6.0%,p= 0.10, Cohen’s d = 1.4).ConclusionsThe addition of acute heat stress to HIIE elicited additive adaptations in only cardiovascular function compared to HIIE alone in active young adults in temperate conditions, thus providing evidence for its effectiveness as a strategy to amplify exercise-induced cardiovascular adaptations.