SummaryAllometry, defined as the relationship between the growth rates of organs to the weight of the whole body (38), was used to study the effect of age on the degree of compensatory renal growth (CRG) in the mouse. The normal growth of the kidneys relative to body weight (BW) was determined in animals between 5 to 50 days of age. In one group, nephrectomy and sham operations were performed at 5, 15, and 35 days of age. The remaining (“renopri-val”) kidney was removed 15 days postnephrectomy. In a second group, nephrectomy was performed on 5-day-old animals, the renoprival kidney being removed after 30 or 45 days. Regression equations were calculated by least-squares after logarithmic transformation and different groups were compared by analysis of covariance. The regression equation for the control kidney was kidney weight (KW) = 0.0093 BW0.86(r = 0.96). The regression for renoprival kidneys in females was KW = 0.0142 BW0.83(r = 0.96) after 15 days and, in comparison, was not significantly different after 30 to 45 days. The intrval between control and renoprival regressions was equivalent to a difference of ≃43% KW. In male mice, the regression for renoprival kidneys after 15 days was KW = 0.0103 BW0.96(r = 0.98) and was not significantly different after 30 to 45 days. This study suggests that in young mice the time required for complete CRG may be a maximum of 15 days and that the amount of CRG does not depend on the age at operation. After CRG, a new equilibrium is reached which is therafter maintained up to a minimum of 50 days of age.SpeculationUse of allometry to study compensatory renal growth suggests that the renoprival kidney grows rapidly until its mass is 70 to 75% of the normal total renal mass, after which its growth rate reaches an equilibrium with body weight. This precise degree of growth, which is known to occur mostly in the cortex, suggests the existence of a negative feedback loop regulating overall kidney weight when cortical mass (or function) is restored.