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Compression of the neck of the sitting subject caused increase in urinary sodium output without change in glomerular filtration (creatinine clearance) or apparent cardiac output (electrokymograph). The possible existence of an intracranial mechanism sensitive to alterations in volume of extracellular fluid and regulating the volume of extracellular fluid is suggested. P REVIOUS studies from this laboratory,',' have indicated: (a) that under comparable conditions the urinary excretion of sodium, like that of chloride' and of water,4 is less in the sitting than in the recumbent position, and (b) that compression of the neck of the sitting subject may cause increase in sodium excretion. Since renal clearances were not measured, the previous work provided no evidence concerning the relative importance of alterations in filtration and in reabsorption.The purposes of the present communication are to extend the previous observations, particularly in relation to: (a) investigations of glomerular filtration and cardiac output, (b) studies of the excretion of chloride and of potassium, as well as of sodium and water, and (c) measurements during congestion of the head under different circumstances (induced by tilting the body or by compression of the neck of the recumbent subject). It is hoped that such observations may furnish information concerning some of the mechanisms regulating sodium excretion under physiologic conditions, in order that these mechanisms may later be investigated in patients with circulatory disorders. METHODS The subjects were normal males, aged 22 to 30. Various attempts were made to maintain