Abstract In hot conditions, cattle can become unproductive, overheat or die. To assess the effect of a particular set of environmental conditions on an animal, the thermal balance is modelled and coded into a user-friendly software package. Air temperature, humidity, wind speed and radiation, together with metabolic heat production, are used to evaluate the thermal balance. Three biological cooling mechanisms are included: reduction of thermal resistance of body tissue, sweating to increase the latent heat loss from the skin and panting to increase heat loss from the respiratory system. The body temperature rises if the cooling mechanisms cannot dissipate sufficient heat. The model is used to examine the effects of conditions likely to be experienced by cattle in southern UK. The results show that high yielding animals would be able to maintain thermal balance in normal conditions but, in hotter conditions, feed intake would be reduced and production would fall.