1. In ten patients with renal cell carcinoma (Grawitz tumour) a selective kidney function study was performed with the intention of measuring the effect of the tumour upon the function of the parenchyma surrounding the tumour. 2. The excretion fraction of sodium and chloride and the osmolal clearance per unit of glomerular filtrate were significantly less and the urinary concentration of inulin and para-amino-hippuric acid were significantly greater in the affected than in the non-affected kidneys. In three patients with poorly vascularized tumours the difference between the affected and the non-affected kidney was insignificant or small. 3. The findings indicate hypoperfusion of the parenchyma surrounding the tumour. The hypoperfusion may be caused by diversion of circulating blood from the parenchyma to the tumour. 4. There was no correlation between the occurrence and degree of hypoperfusion and the blood pressure.