1. The effect of infusion of iso-osmotic saline on renal haemodynamics was studied to determine if the natriuresis that follows is associated with parallel and constant changes in renal cortical or outer medullary blood flow.

2. Blood flow was determined by external monitoring of the washout of 133Xe and the clearance of p-aminohippurate at two rates of saline loading in anaesthetized dogs.

3. After moderate saline loads (40 ml/kg body weight), increase in blood flow was limited to the renal cortex. After large saline loads (80 ml/kg body weight), a significant increase in outer medullary flow occurred in addition to increased cortical flow.

4. Changes in outer medullary blood flow as measured by the washout of 133Xe agreed with those predicted from the extraction of p-aminohippurate.

5. The extent of natriuresis did not consistently correlate with changes in either cortical or outer medullary blood flow. Whereas increases in renal blood flow were associated with a brisk natriuresis, it appears there is no simple relationship between increased cortical or outer medullary flow and sodium excretion.

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