On male Sprague Dawley rats isotonic NaCl during control and 1 M NaCl were infused either into the carotid artery or i.v. Glomerular filtration rate, sodium output and plasma renin activity were determined. Five of 19 animals reacted with no increase in sodium output for 1 M NaCl and are treated as a different group from the animals reacting with an increased sodium output. For the animals reacting with an increased sodium output a decrease in plasma renin activity was found together with an increase in glomerular filtration rate. In both groups the plasma sodium was constant. The animals not reacting with an increased sodium output had a higher initial plasma renin activity, which did not change during 1 M NaCl infusion. The responses were equal for both intracarotid and i.v. infusions but with a somewhat longer delay before the response occurred with i.v. infusions. These results might be explained by a central nervous effect and by a direct renal effect. Also during 1 M NaCl infusion a possible extracellular volume expansion, derived from water withdrawal from the cells into the extracellular space might occur.