The renin inhibitory activity of 2-[4-(4'-chlorophenoxy)phenoxyacetylamino]ethylphosphorylethanolamine (PE-104) was examined in vitro and in vivo. PE-104 inhibited the reaction between dog renal renin and homologous plasma angiotensinogen. The Ki value was 2 mM and the inhibitory mode was competitive and reversible. Data concerning the relationship between renin inhibitory activity and the chemical structure indicated that the whole structure was required for inhibitory activity of PE-104. PE-104 did not inhibit the caseinolytic activities of pepsin, papain and trypsin at 10 mM, the dose of which inhibited renin activity by more than 80%. In normotensive rats, infusion of PE-104 (20 mg/kg/min) abolished increases in blood pressure, plasma renin activity and plasma angiotensin I concentration after injection of renin. In two kidney model renal hypertensive rats, infusion of PE-104 resulted in decreases in blood pressure, plasma renin activity and plasma angiotensin I concentration.