In eighteen patients with hypertension of presumed renal origin the haemodynamics of the kidney were studied by arterial pressure recording, blood flow measurement and selective arteriography before and after a splanchnic block. The material was divided into kidneys with and kidneys without arterial stenosis. In these patients with hypertension a positive correlation was found between mean arterial pressure and renal vascular resistance both before and after the splanchnic block. In patients with renal arterial stenosis the change in relative renal vascular resistance was negatively correlated to the initial resistance, implying that in kidneys with a high initial vascular resistance the resistance decreased to a relatively greater extent than in kidneys with a low initial resistance. At high blood pressures ischaemic areas in the kidney were found. The volume of these areas is dependent upon sympathetic tone, such that a high sympathetic tone results in ischaemia of a larger volume of the kidney. This applies both to kidneys with and to those without arterial stenosis. The results of this investigation thus support the assumption that in the presence of hypertension a kidney is under the control of a sympathetic tone that may form a part of the pathogenetic process in hypertension.