The problem of feed utilization and heat transfer as it applies to a ruminant is presented. A simple mathematical model of the situation is developed and used to predict the amount of heat that can be disposed of by an animal in terms of its size, ambient temperature, wind speed and water available to it for evaporative heat loss (perspiration). In terms of known digestion efficiency, it is shown that animals receiving excessive heat energy (e.g. exposed to sunlight and/or raised in temperatures above their blood heat) may have to reject their food because they are unable to dispose of the associated heat. Some of the implications of this with regard to protein/energy balance in tropical feed strategies are discussed. The particular advantages of enhanced microbe efficiency in the rumen and the use of by-pass protein for ruminant productivity in the tropics are suggested.