Both isotope clearance studies in dog and man and morphometric findings in the rabbit demonstrate that compensatory changes in the remaining kidney start immediately after contralateral nephrectomy and are completed no more than 3 months later. In the meantime, there is a transitorily exceeding rise of ERPF, GFR, glomerular capillary diameter and volume of the glomerulus, evidence of a "reactive hyperemia". 8-12 weeks after unilateral nephrectomy, clearance values of both men and dogs have fallen to a constant level corresponding to 70 per cent of the value of two organs. Loss of function in one kidney is compensated by the contralateral organ in the presence of the damaged partner. The removal of a non-functioning kidney usually will not result in a rise of function in the remaining organ.