The mechanism causing renal vascular flow to vary less than proportional to changes in arterial-venous pressure gradient, was studied in isolated dog kidneys perfused with whole blood and with cell-free colloidal solutions. This autoregulation of renal flow rapidly deteriorated along with vascular reactivity to drugs when oxygenated polyvinyl-pyrrolidone-Locke solution was used for perfusion. This deterioration was prevented by the addition of plasma to the colloidal perfusate.During the first 2 seconds of suddenly raised arterial pressure, renal flow normally increased proportionately or slightly more than proportionately to the increase in arterial pressure; intrarenal venous pressure, needle tissue pressure and kidney weight rose simultaneously. During the next 4 seconds, increasing vascular resistance upstream from the intrarenal veins caused parallel reductions in renal flow, intrarenal venous pressure, needle tissue pressure and at times kidney weight. After brief rhythmical changes in prevenous segmental resistance, flow became steady to show intense autoregulation, while intrarenal venous pressure and needle tissue pressure remained relatively low.This genuine autoregulation of renal flow was abolished by cooling kidneys to 3 to 10 C, and by treatment with chloral hydrate and with procaine in concentrations rendering the smooth muscle of the renal blood vessels relatively inert to direct drug stimulants. On the other hand, at temperatures of 3 to 10 C., and usually with chloral hydrate treatment, a factitious and passive type of flow autoregulation was observed, caused by the effects of abnormally high tissue pressures.Renal flow autoregulation was not appreciably impaired by anesthetization of the intrarenal nerves by procaine in concentrations which did not simultaneously depress vascular smooth muscle reactivity. Yohimbine induced sympatholysis did not impair autoregulation, and Dibenzyline treatment to intrarenal sympatholysis depressed only slightly autoregulation of renal flow. It was not inhibited by γ-aminobutyric acid.Anoxic perfusion which did not appreciably depress the reactivity of intrarenal autonomic ganglia, impaired autoregulation moderately. The loss of autoregulation of renal flow, accompanied by vasoconstriction following severe hemorrhage in the kidney donor dog, was slowly reversible upon perfusion of the subsequently isolated kidney and was related to smooth muscle contracture within the arterial-arteriolar vasculature.It is concluded, that myogenic vasomotion in the renal arterial-arteriolar tree in response to the level of transmural vascular pressure is the fundamental cause of genuine renal circulatory autoregulation. It is furthermore suggested that the myocytes of the juxtaglomerular apparatus may act as myogenic pacemakers in the vasomotion responsible for the essentially perfect autoregulation of the normal kidney.