The quantification of the relationship between feed composition and the efficiency of protein utilisation is a critical step in any model intended to predict the responses of an animal to its diet as rates of protein and lipid retention. Current descriptions of this relationship propose that the efficiency of protein utilisation (ep) is constant and independent of energy supply (Stranks et al, 1988), that ep is a function of protein intake only (ARC, 1981) or that it is a complicated function of the supply of energy at constant protein intake (Fuller and Crofts, 1977). The objective of the two experiments described here, was to define the form of the relationship between feed composition and ep, by using suitable experimental designs.