Abstract: AIMTo study the vasorelaxation action of oxyphenamone (Oxy) and its mechanism. METHODS The contractile response of isolated rabbit renal, femoral and mesentery artery preparations was determined. RESULTS Oxy was shown to inhibit the contractile force of renal, femoral and mesentery arteries induced by phenylephrine in a concentration dependent manner. The vasorelaxation produced by Oxy was not attenuated by removal of the endothelium. Oxy (10^-6～10^-4 mol·L^-1) relaxed the contractions induced by KCl 30 mmol·L^-1 as well as KCl 80 mmol·L^-1, but the contraction curve of KCl 80 mmol·L^-1 was shifted significantly to the right. Oxy in lower concentration (10^-6 and 5×10^-6 mol·L^-1) increased the contractions induced by Ang II, and in middle concentration (10^-5 mol·L^-1) it did not affect the contractions induced by Ang II. Whereas in higher concentration (5×10^-5 mol·L^-1) it obviously inhibited the contractions induced by Ang II. CONCLUSIONOxy showed significant vasorelaxation to various vascular preparations, and its vasorelaxation action is endothelium independent. The mechanism of its vasorelaxations seems to be related with Ca²⁺ activated K⁺ channel (K_Ca channel) and Ca²⁺ channel in vascular smooth muscle cells but its true mechanism needs further study.