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Summary Enzymatically modified isoquercitrin (EMIQ) is a water-soluble glycoside of quercetin produced from rutin by enzymatic treatment. We investigated the anti-hypertensive effect of orally administered EMIQ in spontaneously hypertensive rats (SHR). The systolic blood pressure (SBP) in SHR administered EMIQ at a dose of 3 and 26 mg/kg/d was significantly lower than that in the control group on d 22, 36 and 50 of administration. The effect of EMIQ (26 mg/kg/d) was higher than equimolar administration of quercetin. Diltiazem administered as a positive control also suppressed the increase in SBP, and the effect was stronger than that of EMIQ. In the control group, the mean values of mean blood pressure (MBP) and diastolic blood pressure (DBP) were increased after the start of administration. Although diltiazem suppressed the increase in MBP, no significant changes were observed in the EMIQ groups. Compared with the control group, EMIQ groups showed the incidental changes of MBP and heart rate on day 22 of administration only. These results indicate that EMIQ suppressed the increase in SBP in SHR dose-dependently, and was more effective than the aglycone quercetin. It was also speculated that EMIQ showed higher antihypertensive effect than quercetin due to the high bioavailability, and the mechanism of SBP suppression is possibly through the improvement of endothelial NO production. In conclusion, our results suggest that EMIQ shows possibility as a naturally-derived safe food material which has an antihypertensive effect. Key Words enzymatically modified isoquercitrin, flavonoid, hypertension, blood pressure, spontaneously hypertensive rats Hypertension, which is closely related to the lifethreatening diseases arteriosclerosis and cardiovascular disease, is gathering increasing concern worldwide. There are many evidence-based antihypertensive medicines, such as calcium antagonist, angiotensin-converting enzyme (ACE) inhibitors and sympathetic blocking agents. However, with the recent increase of lifestylerelated diseases, there is growing concern about their prevention by dietary modification. There are many factors involved in hypertension, such as insufficient protein intake, excess intake of NaCl, and shortage of calcium and zinc, which are attributed to a disturbed nutritional status.Recently, oxidative stress due to reactive oxygen species (ROS), such as the superoxide radical and hydroxyl radical, has been reported to be involved in the development of hypertension ( 1 , 2 ). Negishi et al. reported an increase in the oxidative stress marker, urine 8-OHdG in a hypertensive patient compared with normotensive subjects suggesting that hypertensive patients have a high level of oxidative stress ( 3 ). Nitric oxide (NO) has a vasodepressor effect, and is closely related with oxidative stress ( 4 ). The increased ROS can decrease the halflife of NO. Certain reports suggested that the increase of ROS and simultaneous decrease of NO and antioxidants such as SOD and vitamin E occurs in essential hypertension ( 2 , 5...