The distribution of Na, K, and water in the rat tail artery was studied with a view to further characterizing the distribution of Na within the inulin-inaccessible phase of a typical small artery. Two components of Na can be demonstrated by rewarming precooled, Na-loaded arteries. One component is liberated within minutes of rewarming, unassociated with any K exchange and unaffected by iodoacetate blockade. The larger component is liberated slowly over hours, usually associated 1:1 with the converse movement of K and metabolically driven. The metabolic extrusion of Na can proceed, however, even if K influx is blocked.