A novel angiotensin-converting enzyme (ACE) inhibitory peptide was isolated and purified from chicken bone extract by enzymatic digestion. The peptide was defined as an ACE inhibitor, and it demonstrated antihypertensive activity following oral administration to spontaneously hypertensive rats (SHRs). The results of this study suggest that peptides derived from an extract of chicken bones, administered orally, have the ability to reduce the blood pressure of SHRs significantly over a short period of time (3 h). Moreover, the blood pressure then remains low for 3 h. This peptide derived from chicken bones may therefore have great value as a short-term remedy for chronic conditions such as high blood pressure. The amino acid sequence of the peptide was YYRA (Tyr-Tyr-Arg-Ala), which was the origin of the Ig heavy chain V region (27-30 position). The IC50 value of its synthetic peptide was 33.9 mu g/mL. We suggest that the ACE inhibitory and antihypertensive peptides derived from chicken bone extract may contribute to develop physiologically functional foods or improve food functionality.