SummaryA theoretical integrative model was developed to determine the heat balance of horses working in a given environment. This model included the following parameters: metabolic heat gain, solar heat gain, evaporative heat loss due to sweating, respiratory tract heat loss, radiation from the body and heat gain or loss due to convection and conduction.The model developed in this study includes an unique approach for estimating heat loss via evaporation of sweat from the animal's skin surface. Previous studies modelling evaporative heat dissipation were based on the volume of sweat loss. While it is known that the ambient conditions affect evaporation rate, these effects have not been adequately described. The present model assumes the horse's skin surface is adequately represented by a body of water and it describes the interaction of that water body with the atmosphere.It is assumed that sweat has thermodynamic characteristics equivalent to distilled water. Sweat, however, has high electrolyte and protein concentrations and anecdotal evidence has shown that the thermodynamic characteristics may be significantly affected. Further research is, therefore, required to confirm these characteristics for equine sweat.The model describes all factors known to affect the thermal balance of the horse working in a given environment. The relative significance of the various variables on the whole integrative model has been illustrated. The effect of ambient temperature and humidity on the evaporative heat loss, the most significant and critical avenue of heat dissipation, is defined and quantified. The model illustrates clearly how increasing relative humidity limits evaporative heat loss, which can be further compromised when horses exercise on treadmills with no air movement.