Abstract The restitution of blood volume and plasma protein after 10 per cent hemorrhage was studied in intact dogs and in dogs subjected to adrenalectomy and infused with cortisol at basal or physiologically increased rates. The results suggest that increased extracellular osmolality, mediated by an increased secretion of cortisol, mediates, in turn, the restitution of protein and plasma volume. The results are best explained by the hypothesis that after hemorrhage, as a direct result of an increased secretion of cortisol, extracellular osmolality increases. A shift of fluid from cells to interstitium follows, increasing the interstitial pressure. This latter increase leads to an increased return of protein through the lymphatics and to an increased return of fluid through both lymphatics and capillaries.