The aim of this research is to understand in detail how inhibitors (and substrates) interact with the active sites of acid proteases, specifically pepsin and the acid protease of Rhizopus chinensis. The work has importance for a full understanding of the mechanism of action of these enzymes, and for design of specific inhibitors of renin of control of hypertension. The method of study involves mainly 1H NMR studies of pepstatin and pepstatin analogs binding to porcine pepsin. Specifically deuterated analogs will be used, as well as spin-labeled pepstatin derivatives. The objective is to provide a detailed molecular description of the interaction of strong inhibitors with amino acid groups in the active site of acid proteases.