SUMMARY1. The effect of changes in the haematocrit of blood perfusing the kidney on its intrarenal distribution was studied in dogs.2. Two types of preparations were employed. (i) In the isolated perfused kidney evidence is presented that flow in the autoregulating preparation represents predominantly cortical flow while flow in the 'low flow nonautoregulating' kidney reflects medullary flow. (ii) In the intact kidney renal blood flow rate and its intrarenal distribution was studied by the injection of 133Xe into the renal artery and measuring its clearance from the kidney by an external counter.3. In both types of preparation cortical flow was found to be independent of changes in P.C.V. but medullary flow varied inversely with haematocrit.4. A change in the haematocrit of the perfusing blood leads to alteration of its viscosity. It was argued that an increase in viscosity must lead to a reduction in the resistance of the cortical afferent arterioles but that medullary afferent arterioles were not able to respond in this manner.5. These findings demonstrate that changes in total body haematocrit cause a redistribution of blood flow between renal cortex and medulla.

1. Acute changes in haematocrit were produced by exchange transfusion of dextran-in-saline or packed red cells.2. There were no significant changes in glomerular filtration rate, blood pressure, central venous pressure, heart rate, respiratory rate, blood volume or extracellular fluid volume following the exchange transfusions.3. Urine volume increased after haemodilution but decreased after haemoconcentration.4. The diuresis after haemodilution occurred despite an infusion of ADH or alcohol. Thus it could not be attributed to a change in circulating ADH level.5. There were two types of diuresis. The ;water diuretic' response was characterized by an increase in free water clearance with a reduction in urinary sodium concentration; the ;sodium diuretic' response by an increase in urinary sodium concentration but no change, or a fall, in free water clearance.6. The results were related to changes in medullary osmotic gradient found by other workers to occur when medullary blood flow rate is altered.