Arterial plasma renin activity (PRA) was measured using radioimmunoassay techniques in pentobarbital-anesthetized dogs. Acute saline loading (40–60 ml 150 mM NaCl/kg body weight, or 20 ml 300 mM NaCl/kg body weight, i. v.) reduced PRA from control level in 7 dogs. Despite continued increases in filtered and excreted masses of Na, subsequent partial aortic clamping increased PRA in proportion to the degree of reduction in renal perfusion pressure. In 6 other dogs aortic clamping superimposed on total ureteral occlusion led to changes in PRA which were inversely associated with renal perfusion pressure. These results suggest a controlling mechanism for renin secretion sensitive to some hemodynamic paramete