Brown alga (Undaria pinnatifida) was treated with alginate lyase and hydrolyzed using 17 kinds of proteases andthe inhibitory activity of the hydrolysates for the angiotensin-I-converting enzyme (ACE) was measured. Four hydrolysateswith potent ACE-inhibitory activity were administered singly and orally to spontaneously hypertensive rats (SHRs). The sys-tolic blood pressure of SHRs decreases significantly after single oral administration of the brown alga hydrolysates by pro-tease S ’Amano’ (from Bacillus stearothermophilus) at the concentration of 10 (mg protein) (kg body weight)-1. In the 17weeks of feeding experiment, 7-week-old SHRs were fed standard diet supplemented with the brown alga hydrolysates for 10weeks. In SHRs fed 1.0 and 0.1% brown alga hydrolysates, elevating of systolic bloodpressure was significantly suppressedfor 7 weeks. To elucidate the active components, the brown alga hydrolysates were fractionated by 1-butanol extraction andHPLC on a reverse-phase column. Seven kinds of ACE-inhibitory peptides were isolated and identified by amino acid compo-sition analysis, sequence analysis, and LC-MS with the results Val-Tyr, Ile-Tyr, Ala-Trp, Phe-Tyr, Val-Trp, Ile-Trp, andLeu-Trp. Each peptide was determined to have an antihypertensive effect after a single oral administration in SHRs. Thebrown alga hydrolysates were also confirmed to decrease the blood pressure in humans. Brown alga (Undaria pinnatifida) was treated with alginate lyase and hydrolyzed using 17 kinds of proteases andthe inhibitory activity of the hydrolysates for the angiotensin-I-converting enzyme (ACE) was measured. Four hydrolysateswith potent ACE-inhibitory activity were administered singly and orally To spontaneously hypertensive rats (SHRs). The sys-tolic blood pressure of SHRs significant significantly after single oral administration of the brown alga hydrolysates by pro-tease S ’Amano’ (from Bacillus stearothermophilus) at the concentration of 10 (mg protein) ( Kg body weight)-1. In the 17weeks of feeding experiment, 7-week-old SHRs were fed standard diet supplemented with the brown alga hydrolysates for 10weeks. In SHRs fed 1.0 and 0.1% brown alga hydrolysates, elevating of systolic bloodpressure was significantly Suppressedfor 7 weeks. To elucidate the active components, the brown alga hydrolysates were fractionated by 1-butanol extraction andHPLC on a reverse-phase column. Seven Kinds of ACE-inhibitory peptides were isolated and identified by amino acid compo-sition analysis, sequence analysis, and LC-MS with the results Val-Tyr, Ile-Tyr, Ala-Trp, Phe-Tyr, Val-Trp, Ile- Trp, andLeu-Trp. Each peptide was determined to have an antihypertensive effect after a single oral administration in SHRs. Thebrown alga hydrolysates were also confirmed to decrease the blood pressure in humans.