1. After spontaneous subarachnoid haemorrhage in man there is an acute rise in arterial blood pressure. Sodium balance studies were carried out on sixty-four patients with this syndrome and marked sodium retention was found to occur which did not appear to be correlated with a low glomerular filtration rate nor with any consistent change in aldosterone excretion.
2. Effective renal plasma flow was measured by p-aminohippurate clearance in twenty-four patients and renal blood flow calculated from the haematocrit. Renal blood flow was found to be significantly depressed during the first week of the illness. The possibility that the sodium retention was due to changes in proximal tubular reabsorption secondary to alterations in intrarenal pressure is discussed.
3. No change in potassium balance was found despite the frequent occurrence of hypokalaemia. The depression of plasma potassium was closely related to the degree of respiratory alkalosis. Since respiratory alkalosis has been shown in animals to occasion a shift of potassium from the extracellular fluid into the cells, it is postulated that a similar change occurs after subarachnoid haemorrhage in man.