In our continuing search for low molecular weight, human renin inhibitors, a dipeptide derivative, morpholino-naphthyl-acyl-histidyl-cyclohexyl-norstatine (KRI-1314), was newly synthesized and estimated for oral effectiveness. This compound inhibited plasma renin from humans and from Japanese monkeys in vitro, with 50% inhibitory concentrations (IC50) of 4.7 x 10(-9) and 2.4 x 10(-8) mol/l, respectively. The mean blood pressure of sodium-depleted Japanese monkeys was lowered significantly after intravenous injection or oral administration of KRI-1314. The maximum reduction was attained 3 h after oral administration at a dose of 10 mg/kg. Plasma renin activity (PRA) was halved 1 h after oral administration of KRI-1314, and this inhibition persisted for more than 5 h. results suggest that KRI-1314, a potent, orally effective and long-lasting renin inhibitor, may become one of a new class of antihypertensive agent.