Abstract: The aim of this study was to determine the effect of ethanol extract of kucai bulbs on serum nitric oxide (NO) levels in rats, in an attempt to study the mechanism of antihypertensive action. The measurement of NO level was conducted by Griess colorimetric assay. The male Wistar rats were divided randomly into three groups; the first group was treated with kucai extract of 100 mg kg^-1 body weight (kucai-treated group); the second group with isosorbide dinitrate (ISDN) of 0.18 mg kg^-1 body weight (ISDN-treated group) and the third group with vehicle only (control group). The results showed that at 30 min (T₃₀) the mean NO level in kucai-treated group increased significantly (138.69 ± 169.38%) compared to baseline (T₀); similarly, in ISDN-treated group, the mean NO level increased (212.08 ± 140.01%) significantly. At T₉₀, average NO levels in kucai-treated group was significantly higher than those in control group (p = 0.018), while those in ISDN-treated group were not significantly higher than those in control group (p = 0.436). Compared to kucai-treated group, NO levels in ISDN-treated group at T₉₀ was significantly different (p = 0.028). From this study it can be concluded that ethanol extract of kucai bulbs increased significantly serum NO level in experimental rats. Therefore, it is preassumed that antihypertensive effect shown by kucai is, one of which, due to mechanism of vasodilator action. The present results showed that kucai increased NO levels and this might be one mechanism of blood pressure lowering action of kucai.