Heparin is shown to inhibit the in vitro production of angiotensin by renin. The inhibition of renin is much greater in water than in saline incubations, and is independent of the amount of renin present, but dependent on the concentration of the renin substrate. Heparin inhibition thus occurs as a consequence of reversible competition with the renin substrate for the active site of the enzyme. The data indicate that, in salt-free incubations, this inhibition may be quite specific because the binding constant for renin with heparin is nearly of the same order of magnitude as that observed for renin with its natural substrate. This action may possibly be relevant to the frequently described influence of heparin in reducing aldosterone secretion in hypertensive and edematous states. (J Clin Endocr 27: 699, 1967)