On the basis of previous studies in rats, and clinical observations in human renal transplantation and in patients with a renal duplication anomaly, we have postulated two distinct types of renal growth. Compensatory growth is caused by a nephron deficit, while obligatory growth is part of the normal process of growing into adulthood. The former is reversible, and the latter is not. A definitive test of this hypothesis required the transplantation of extra kidneys into baby rats and comparison of their growth with that of normal litter‐mate controls. This showed that there is little if any retardation of obligatory renal growth in spite of the presence of an excess of kidneys in the bab