We have shown that Protease N treated Royal Jelly (ProRJ) and peptides from ProRJ (Ile-Tyr (IY), Val-Tyr (VY), Ile-Val-Tyr (IVY)) inhibited angiotensin I-converting enzyme (ACE) activity and they have an antihypertensive effect in repeated oral administration for 28 d on spontaneously hypertensive rats (SHR). We investigated the contributive ratio of these peptides in ProRJ for antihypertensive effect in single oral administration on SHR. In single oral administration of each peptide and peptides mixture (MIX; IY, VY and IVY) at doses of 0.5, 1 and 10 mg/kg, systolic blood pressure (SBP) of SHR was reduced dose-dependently. This antihypertensive effect was held for 8 h. These results suggest that peptides contributed to the antihypertensive effect of ProRJ. And the contributive ratio of MIX in ProRJ for antihypertensive effect was computed to be about 38%. Therefore it is considered that intake of peptides, as a functional food would be beneficial for improving blood pressure in people with hypertension.