Water and sodium excretion after blood volume expansion under conditions of constant arterial, venous and plasma oncotic pressures and constant haematocrit.

1. The diuretic and natriuretic responses occurring during expansion of blood volume by homologous blood transfusion were studied in homologous kidneys transplanted to the neck of hydropenic dogs that had previously been given deoxycorticosterone acetate and antidiuretic hormone. The experimental conditions ensured constant arterial perfusion pressure, venous pressure, osmotic pressure, haematocrit and plasma oncotic pressure. 2. Moderate but significant increases in urine output, renal sodium excretion, osmotic clearance and tubular sodium rejection fraction were observed; there were no significant changes in glomerular filtration rate, renal blood flow, postglomerular haematocrit and postglomerular plasma protein concentration 20 and 40 min after the end of blood infusion. 3. As the non-hormonal factors known to modulate sodium excretion underwent no significant change, the results are compatible with the proposition that a specific factor (‘natriuretic hormone’) plays a role in the mechanism of natriuresis after blood volume expansion.