Traditional methods of measuring renin depend on activity in conversion of angiotensinogen to angiotensin I. Angiotensin I is converted physiologically to angiotensin II by angiotensin converting enzyme (ACE). Angiotensin II is a potent vasoconstrictor, stimulating cells of the glomerulus to produce aldosterone. A decrease in perfusion pressure to the juxtaglomerular apparatus and a negative sodium balance are the primary stimuli for renin release. The Direct Renin method provides more accurate analytic reliability for renin measurement than activity-based methods. The method permits the measurement of both active and total renin by means of a renin inhibitor that opens the active site of the renin enzyme and permits binding by specific antibody to the active site. Quantitation of renin both in the presence and absence of the inhibitor provides an accurate immunological measurement of active renin without relying on a measurement of enzyme activity that is more variable and dependent on substrate, temperature, and stability of the various components.