The allometric-autoregressive quantification of growth and efficiency divides rat growth into 3 different physiological phases. Significant differences between families for some of these parameters existed. Consequently, heritabilities and, phenotypic and genetic correlations of the different parameters were estimated. Significant heritabilities were found for some of the parameters, while the heritabilities for others were close to zero. The correlations between the parameters varied from highly positive or negative to correlations of practically zero. Although based on few animals, the heritability estimates appear to be reasonably acceptable, both statistically and in their biological context. The heritability estimates which approach zero for some parameters are discussed in the light of possible homeostatic effects and the implications of the different estimates are evaluated.