Repeated exercise-heat exposures, known as heat acclimation (HA), are often implemented as an intervention to attenuate decrements in physiological strain and exercise tolerance prior to work in normothermic and hot, humid conditions. The fundamental potentiating stimuli for thermoregulatory adaptation are repeated, significant rises in core temperature. Targeting of a specific core temperature is known as isothermic, or controlled hyperthermic HA. Different methods of modulating the exercise component of isothermic HA have been implemented, with prescription previously based upon either peak oxygen uptake (VO 2peak), power, or subjective ratings of perceived exertion or thermal sensation. Interestingly, metabolic heat production, a measure to determine changes in core temperature, has not been used to prescribe isothermic HA. The aim of this study was to determine the relationship between the rate of rectal (core) temperature (Trec) increase, and different methods for prescribing workload during an acute exercise-heat exposure, with the objective of trying to refine the prescription of isothermic HA workloads.