SummaryThe biochemical action of renin on its substrate (angiotensinogen) leads to the formation of angiotensin I which is converted by angiotensin converting enzyme (kininase II) into the vasoactive peptide angiotensin II which has many biological functions. Another angiotensin converting enzyme (tonin) forms angiotensin II directly from renin substrate.Indirect means of assessing the plasma renin level have been developped such as plasma renin activity (PRA) and renin concentration assayed after acidification of the plasma to pH 3.3 (PRC3.3). The PRA and PRC3.3determinations give concordant findings in normal subjects on a regular and a low sodium diet and in hypertensive patients on a regular diet and/or after sodium-volume depletion. The relationship between PRA or PRC3.3on the one side and urinary sodium excretion, age or plasma aldosterone concentration on the other side is of the same magnitude. However PRA and PRC3.3determinations can give discordant findings in conditions such as influence of beta-blockade, effect of exercise, renal vein plasma renin levels, infusion of saralasin and females on oral contraceptives.The diagnostic and prognostic value of plasma renin determinations are also reported: hyperaldosteronism, renovascular hypertension, predictive value of renin determinations in the hypotensive effect of beta-blockers or diuretics, renin secreting tumors and renin as risk factor for stroke and heart attack.