Nori oligopeptide (NOP), which is obtained by enzymolysis of nori (Porphyra yezoensis) with pepsin, has angiotensin-I converting enzyme (ACE) inhibitory activity and shows an antihypertensive effect when administered as a single dose to spontaneously hypertensive rats (SHRs) . This study was performed to identify the antihypertensive substance within NOP and to examine its mechanism of action. After separating the material derived by enzymolysis (crude NOP material) of nori into a NOP fraction, an external dialysate solution fraction (ash fraction) and an internal dialysate solution fraction (non-fibrous carbohydrate and dietary fiber fraction), we administered each of them to SHRS by mixing them in the diet for 28 days. Only the NOP fraction significantly lowered the systolic blood pressure, proving that the antihypertensive effect of long-term administration was due to NOP. The effective antihypertensive single dose of the ACE inhibitory peptide Ala-Lys-Tyr-Ser-Tyr (AKYSY) present in NOP to SHRs was 0.2 mg/kg. On the other hand, the effective antihypertensive dosage of crude NOP material was 200 mg/kg, suggesting that the antihypertensive effect of NOP was due primarily to AKYSY, since 200 mg of crude NOP material contains 0.135 mg of AKYSY. It was also observed that administration of AKYSY lowered ACE activity in the aorta and lung of SHRs.