The enzyme renin is the initiator of a series of steps that ultimately leads to the generation of angiotensin II, a potent pressor peptide that also has both a direct and indirect role in renal sodium conservation. Renin's action on its protein substrate, angiotensinogen, is inhibited by specific antibody, a peptide produced by actinomyces, pepstatin and its derivatives, and by peptide analogs of a segment of the endogenous substrate. These inhibitors vary considerably in specificity, inhibitory constant, and utility for in vivo studies.