SUMMARY Plasma renin activity (PRA) increases after acidification of plasma or exposure of plasma to cold temperatures. The purpose of the present study is to evaluate the hypothesis that these increases of PRA reflect inactivation of circulating renin inhibiting factors. In each of eight normotensive subjects, mean PRA increased (p < 0.01) after acidification of plasma by acid dialysis (98%), after acidification of plasma by addition of HCI (47%), or after exposure of plasma to −4°C for 5 days (155%). Renin substrate-free plasma was obtained by passing plasma over Sephadex G-50-40. Both untreated plasma and the substrate-free extract of plasma inhibited the enzymatic activity of renin (p < 0.01); little or no inhibition occurred after addition of acidified plasma or the protein-free extract of acidified plasma to renin-renin substrate. After addition of exogenous renin to cold-exposed plasma, the rate of angiotensin I generation was greater than that in control plasma (p < 0.05), although renin substrate did not differ. These observations suggest that denaturation of a circulating renin inhibitor may contribute to increased PRA after acidification or cold exposure of plasma. In a separate experiment, the addition of polyunsaturated free fatty acids (linoleic and arachidonic) to renin-renin substrate inhibited angiotensin production, although no inhibition occurred after dialysis of these fatty acids against an acid buffer (pH 3.3). Prolonged exposure of linoleic acid to cold temperatures did not affect its capacity to inhibit renin. Thus, loss of long chain, unSaturated free fatty acids may contribute to increased PRA after acid dialysis of plasma but not to increased PRA after exposure of plasma to cold temperatures.