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In our previous study, an antihypertensive protein hydrolysate was prepared from sardine.This study aimed to investigate the composition of sardine protein hydrolysate (SPH) and its in vivo antihypertensive effect.SPH was separated sequentially with ultrafiltration and size exclusion chromatography.Fractions with high angiotensin-Ⅰ-converting enzyme (ACE) inhibitory activity were further analyzed with RP-HPLC and amino acids analysis.Then, SPH was individually oral administrated to spontaneously hypertensive rats (SHR) and normotensive wistar kyoto rats (WKY) for 4 wk.After treatment, the systolic blood pressure (SBP), organ index, oxidative status, serum ACE activity, and serum angiotensin-Ⅱ (ANG-Ⅱ) of all the rats were determined.According to the separation and analysis results, three main fractions with high ACE-inhibitory activity were obtained with different amino acids composition.The animal experiments results showed that SPH could significantly reduce SBP (P < 0.05 or P < 0.01) within 4 h after a single oral administration.After a chronic oral administration, a steady state of SBP in SHR rats was attained.The heart weight index and left ventricular weight index were significantly reduced (P < 0.05) in SPH-treated SHR rats.The malonyldialdehyde (MDA) levels in organs and serum, serum ACE activity, and serum ANG-Ⅱ concentration in SPH-treated SHR rats were significantly lowered (P < 0.05 or P < 0.01) as compared to their controls.Meanwhile there was no significant effect (P > 0.05) on those parameters in WKY rats.These results indicate that SPH can decrease blood pressure in SHR rats and not in WKY rats.