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SUMMARY The presence of acetone-soluble renin inhibitors in normal plasma bas been proposed to explain the variation of plasma renin reactivity (PRR) in samples from normotensive and hypertensive subjects. In our experience, acetone extraction decreased PRR in relation to unextracted control values, an observation which is not consistent with the circulating lipid-renin inhibitor hypothesis. Exposure to acetone at −40°C for 1 minute invariably denatured some endogenous angiotenslnogen. The PRR in extracted and unextracted plasma was positively correlated with the concentration of available angiotensinogen, r = 0.955 (p < 0.05), and r = 0.964 (p < 0.01), respectively, but the addition of exogenous substrate did not uniformly increase PRR in acetone-treated plasma above control values. These data argue against the use of acetone extraction to demonstrate the existence of circulating lipid-renin inhibitors. Acetone removed 14% to 25% of the normal plasma lipids and although the extract contained most of the major lipid classes, neutral lipids were the most abundant (73% by weight). The presence of acetone-soluble phospholipids appeared to increase angiotensln I formation in the partially purified renin-angiotensinogen system, but phospholipids interfered with the radiolmmunoassay and resulted in an overestimation of angiotensln I. Plasma neutral lipids decreased in vitro renin activity by 13% (p < 0.025) but this degree of inhibition suggests that lipid-renin interactions may have minimal in vivo physiological significance. In contrast to previous reports, we found the correlation between PRR and endogenous angiotensinogen in normotensive and hypertensive plasmas to be statistically significant (r = 0.643, p < 0.01). Inactivated human angiotensinogen was also shown to be an inhibitor of renin in vitro. This effect could have possibly influenced PRR values that were determined by others in the presence of inactivated angiotensinogen.