The aim of the present study was to investigate the anti-hypertensive and angiotensin-converting enzyme (ACE) inhibition effects of soyabean protein hydrolysate in spontaneously hypertensive rats (SHR). Soyabean protein hydrolysate was prepared by peptic hydrolysis and was added into the feed of SHR (0 % for the S0 group, 0·5 % for the S1 group, and 1 % for the S2 group) for 12 weeks. Systolic blood pressure and mean blood pressure of the S1 (164·3 (sem 4·7); 128·0 (sem 5·0) mmHg) and S2 (156·8 (sem 1·6); 120·8 (sem 3·4) mmHg) groups were significantly lower than those of the S0 group (199·4 (sem 5·2); 158·3 (sem 7·0) mmHg) at the end of the study. In the analysis of ACE activity, plasma and heart ACE activities of the S1 and S2 groups were significantly lower than those of the S0 group, and there were no significant differences in aorta, kidney, and lung ACE activities among all SHR. Soyabean protein hydrolysate had no significant effect on plasma lipids, electrolytes, or on left ventricular wall or aorta wall thickness. The results suggest that the long-term administration of soyabean protein hydrolysate might retard the development of hypertension in SHR by its inhibitory effect on ACE in vivo.