1. The effect of infusion of isotonic saline on the circulation in the renal cortex in the dog was investigated by an external counting technique involving measurement of the transit times for 85Krypton and 131I-labelled albumin after rapid injection into the renal artery.

2. During saline infusion superficial renal cortical blood flow and overall cortical blood flow rose by 23 and 15%, respectively. There was a 6% rise in the ratio superficial cortical blood flow to overall cortical flow, which however, was not significant.

3. Resistance to flow through cortex decreased.

4. Mean transit time for plasma through cortex decreased from an average of 2·9 sec in the control period to 2·6 and 2·1 sec during saline infusion.

5. Renal cortical blood volume, as estimated from the cortical blood flow and the mean transit time for plasma, was virtually unchanged.

6. These studies indicate that the decrease in resistance to flow during acute isotonic saline infusion is probably caused by a dilatation of the resistance vessels only. No significant redistribution of blood flow in cortex takes place; this, however, does not exclude regional changes in glomerular filtration rate.

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