1. The influence of intrarenal blood-flow distribution and plasma transit-time through the renal cortex on the reduction in p-aminohippurate (PAH) extraction (EPAH) induced by renal vasodilator agents was evaluated in anaesthetized dogs by an indicator-dilution technique.

2. Increases in renal blood flow during graded intravenous iso-osmotic saline or hyperosmotic mannitol infusions and during renal arterial infusions of acetylcholine or prostaglandin E1 were consistently associated with reduced EPAH.

3. The cortical fraction of renal blood-flow increased during vasodilatation induced by saline or acetylcholine but was unchanged during mannitol or prostaglandin infusions. Cortical blood volume (CBV) increased with saline infusion but was reduced with mannitol. Changes in CBV with acetylcholine and prostaglandin E1 were insignificant. No correlation could be found between changes in EPAH and changes in intrarenal blood flow distribution.

4. A high degree of correlation (r = 0·81) was noted between EPAH and the simultaneously determined mean cortical plasma transit-time. A significant correlation existed with pooled data for all infusions as well as with individual drug infusions.

5. We interpret these data to indicate that a shortened transit-time of plasma through the cortex, indicative of an increased velocity of cortical plasma flow, is the most important determinant of the lowering of EPAH during renal vasodilatation.

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