Twelve surgically removed human kidneys (mainly tumor kidneys) were investigated. The investigations comprised perfusion criteria (perfusion flow, perfusion pressure, perfusion resistance, electrolyte equilibration). During perfusion of the kidneys with HTK solution, the perfusion resistance was nearly three times as high in human kidneys as in canine kidneys perfused under the same conditions in previous studies. Beside possible species differences the raised perfusion resistance may be explained by the greater trauma to the human kidneys due to the surgery, the primary ischemic stress which cannot be avoided clinically and the often nonoptimal initial diuresis. Nevertheless definitive perfusion is possible under clinical conditions although pronounced increases of perfusion resistance may occur. As indicated by the raised perfusion resistance of human kidneys under clinical conditions as compared with canine kidneys in an experimental model, electrolyte equilibration of human kidneys was protracted. For this reason, a duration of perfusion of at least 10 min is necessary in clinical application of HTK solution, i.e., longer than in animal experiments.