The PSU HEAT protocol has been used to determine critical environmental limits, i.e., those combinations of ambient temperature and humidity above which heat stress becomes uncompensable and core temperature rises continuously. However, no studies have rigorously investigated the reliability and validity of this experimental protocol. Here, we assessed the 1) between-visit reliability and 2) validity of the paradigm. Twelve subjects (5 M/7W; 25 ± 4 yr) completed a progressive heat stress protocol during which they walked on a treadmill (2.2 mph, 3% gradient) in a controllable environmental chamber. After an equilibration period, either dry-bulb temperature (T(db)) was increased every 5 min while ambient water vapor pressure (P(a)) was held constant (T(crit) experiments) or P(a) was increased every 5 min while T(db) was held constant (P(crit) experiments) until an upward inflection in gastrointestinal temperature (T(gi)) was observed. For reliability experiments, 11 subjects repeated the same protocol on a different day. For validity experiments, 10 subjects performed a T(crit) experiment at their previously determined P(crit) or vice versa. The between-visit reliability (intraclass correlation coefficient, ICC) for critical environmental limits was 0.98. Similarly, there was excellent agreement between original and validity trials for T(crit) (ICC = 0.95) and P(crit) (ICC = 0.96). Furthermore, the wet-bulb temperature at the T(gi) inflection point was not different during reliability (P = 0.78) or validity (P = 0.32) trials compared with original trials. These findings support the reliability and validity of this experimental paradigm for the determination of critical environmental limits for maintenance of human heat balance.NEW & NOTEWORTHY The PSU HEAT progressive heat stress protocol has been used to identify critical environmental limits for various populations, clothing ensembles, and metabolic intensities. However, no studies have rigorously investigated the reliability and validity of this experimental model. Here, we demonstrate excellent reliability and validity of the PSU HEAT protocol.