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The blood flow through the dog's kidney tends to remain constant despite changes in perfusion pressure. The early observations on this subject have been reviewed by Winton (1937), and recently Selkurt (1951) and Shipley & Study (1951) have shown that the range of perfusion pressure over which the renal flow remains relatively unchanged lies between approximately 90-190 mm Hg. This phenomenon may be present in the isolated kidney and has been referred to as circulatory 'autoregulation'; as it is not known whether its mechanism is active or passive, its relationship to the intrarenal pressure (I.R.P.) and the effect upon it of cyanide have been studied in an attempt to decide this point. METHODSThe experiments were performed on eight dogs anaesthetized with intravenous pentobarbitone and maintained by intravenous supplements as required. After induction and tracheal intubation, the dogs were suspended on a beam by metal hooks through the skin of the back to facilitate retroperitoneal exposure of the kidney through a flank incision. The renal artery was then cannulated, taking care to avoid, as far as possible, the renal nerves, and the kidney was perfused in situ by a mechanical pump with arterial blood supplied from the carotid artery; the renal venous blood returned to the circulation via the intact renal vein. Perfusion pressure could be altered by changes of the stroke output of the pump and the blood flow measured intermittently with a direct flow-meter; changes in flow were also recorded continuously by an electrical flowmeter. Further details of the operation, the direct flow-meter and the pump have been given in previous papers (de Wardener & Miles, 1952; Miles & de Wardener, 1952).In the three dogs in which the effect of cyanide was studied, the renal artery and vein were cannulated in 8ith, and the kidney then removed from the body and placed in 0-9 %(w/v) sodium chloride solution at 380 C. In order to maintain a constant venous pressure the tube from the renal vein was interrupted at a point, arbitrarily fixed 7 cm above the kidney, and the blood allowed to run into a funnel and thence by gravity to the jugular vein. This perfusion circuit, used during the first part of the experiment, is illustrated in Fig. 1. Later, after autoregulation had been demonstrated, and immediately before cyanide was added to the perfusing blood, the tube from the carotid artery was clamped and the renal venous blood was directed to the flow-meter and pump. In this way the circulation of the kidney became completely isolated from the body. Potassium cyanide, freshly dissolved in sodium chloride solution, was then added in an amount calculated to produce an initial concentration of about 4 m.mole/l.

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