A more deteriorating effect on tumour tissue than on normal renal parenchyma by hypothermic perfusion has previously been observed. The purpose of this study was to further evaluate changes in the perfusate distribution and the viability of normal cortical tissue and of kidney carcinoma during hypothermic perfusion. Tumour-involved kidneys obtained by nephrectomy from 12 patients were studied either with regard to viability, as measured with the Na+-K+ pump function in incubated tissue slices, or with regard to perfusate flow, as analysed with an isotope labelled microsphere technique. Six days of hypothermic perfusion decreased the potassium content in the tumour tissue, while it was mainly unchanged in the cortical tissue. The steady-state level of potassium in incubated slices reflected that the transmembrane K+ influx was unchanged after 6 days of perfusion in normal cortical tissue, whereas a significant reduction was observed in the tumour tissue. The relative perfusate flow in the tumour, when compared to the normal kidney cortex flow, was reduced from 60% after 1 hr of perfusion to 30% after 6 days of perfusion.