Angiotensin I converting enzyme (ACE) inhibitory peptide was isolated from tuna dark muscle hydrolysate prepared by alcalase, neutrase, pepsin, papain, alpha-chymotrypsin, and trypsin, respectively. Among hydrolysates, the pepsin-derived hydrolysate exhibited the highest ACE I inhibitory activity versus those of other enzyme hydrolysates. The structure of the peptide was identified to be Trp-Pro-Glu-Ala-Ala-Glu-Leu-Met-Met-Glu-Val-Asp-Pro (molecular weight 1581 Da) by time of flight mass spectrometry/mass spectrometry analysis, and the IC 50 value of the peptide was 21.6 microM. The Lineweaver-Burk plots revealed that the peptide acts as a noncompetitive inhibitor, and the inhibitor constant ( K i) was calculated as 26.6 microM using the secondary plots. The peptide had an antihypertensive effect according to the time-course measurement after oral administration to spontaneously hypertensive rats. Maximal reduction was detected 3 h after oral administration at a dose of 10 mg/kg of body weight. These results suggest that the peptide derived from tuna dark muscle would be a beneficial ingredient for functional food or pharmaceuticals against hypertension and its related diseases.