Experiments were performed on isolated perfused dog kidneys to determine relationships between tissue pressure, vascular volume, ‘over-all’ and ‘intraorgan’ vascular resistances. Results indicate that autoregulation of renal blood flow is brought about by disproportionately large increases in extravascular pressure. Since extravascular and intravascular pressures are opposing forces there is progressively less vascular distention with increases in arterial pressure as the increments in extravascular pressures approach the increments in intravascular pressures. The effective propellent force of blood flow through the kidney is thought to be the difference between the renal artery pressure and extravascular pressure. Vascular resistances calculated on this basis showed no significant changes throughout the range of autoregulation in four of the six experiments. It is proposed that the extravascular pressure within Bowmans capsule may exceed the renal interstitial pressure, and when the effects of renal extravascular pressures within and without Bowman's capsule are taken into account in the calculation of intrarenal vascular resistance, the pressure-flow relations of the kidney will be similar to those of other vascular beds.