The application of an allometric-autoregressive model for the quantification of growth and efficiency of feed utilization for purposes of selection for efficiency is discussed. To maturity rat growth and efficiency can be divided into 3 growth phases by this model. Each phase is described by a straight line in terms of slope and intercept, and the different growth phases are shown to be associated with physiological processes in the rat. There are significant differences between families for some of the parameters in the first 2 phases. This indicates that the model may have possibilities in the selection of animals for growth and efficiency. The genetics of these parameters will be discussed in a subsequent article.