High-intensity interval training (HIIT) is an effective form of exercise; however, it remains unclear how heat stress, exercise intensity prescription, and recovery mode influence work rate adjustments, thermal and cardiovascular strain, and maximal aerobic capacity (V̇O2max). Three studies investigated these outcomes between the first and fifth round of HIIT (8-min warm-up and rounds of 4-min work×3-min recovery). V̇O2max was measured on separate occasions after the first and final round of HIIT. In Study 1, HIIT was completed in hot (35 °C) and temperate (22 °C) environments using target heart rate (THR) [warm-up and recovery=70% maximal heart rate (HRmax), work=90% HRmax]. To maintain target intensity, considerable decreases in work rate occurred in both conditions, but they were nearly twice as large in the heat, accompanied by elevated recovery HR, higher mean skin temperature, and greater increases in rectal temperature. Following the final round of HIIT, V̇O2max decreased marginally. In Study 2, the HIIT workout was completed in 35 °C at the same THR as Study 1 or a target rating of perceived exertion (RPE) (warm-up and recovery=RPE of 12, work=RPE of 17). To maintain target intensity, work rate decreased 46 W and 30 W in the HR- and RPE-based trials, respectively. Thermal strain was similar between conditions, but elevated cardiovascular strain during RPE-based HIIT corresponded to a larger decrease (15.6%) in V̇O2max compared to HR-based HIIT (6.5%). In Study 3, the HIIT workout occurred in 35 °C and included work at RPE=17 and passive (rest) or active recovery (RPE=12). Thermal strain was similar, but active recovery increased cardiovascular strain and resulted in a larger percent decrease in work rate. V̇O2max declined 11.5% between the first and fifth work intervals regardless of recovery mode. These studies demonstrated all variations of HIIT necessitated meaningful reductions in work rate over time except HIIT based on RPE with passive recovery. Participants ranged from low fit to high fit, but all completed the HIIT sessions. Nonetheless, work rate adjustments and thermal and cardiovascular strain were dependent on the method of exercise intensity prescription and recovery mode and should be considered when prescribing HIIT in the heat.