Extracorporeal circulation depresses renal function. The physiopathological reasons for artificial perfusion affecting glomerular filtration are reported. The extent of the depression is expressed by changes in creatininaemia and creatinin clearance. Two hours after the end of E.C.G., depression reaches its peak but renal function returns to normal within 24 hours of the end of artificial perfusion. The main aid to the rapid return to normal of glomerular filtration is defence of the microcirculation.