Ubiquinone 6.2, 12.5 or 2.5 mg.kg-1 respectively twice iv 24 h and 30 min before coronary artery ligation, ameliorated the ischemic arrhythmia in conscious rats, and there was a close positive correlation between the ubiquinone concentration in myocardium and plasma and its anti-arrhythmic effect. Ubiquinone iv 3.1, 6.2, and 12.5 mg.kg-1 increased, while 25 mg.kg-1 decreased 6-keto-PGF1 alpha, and 12.5 and 25 mg.kg-1 decreased TXB2, which was in accordance with inhibitory effects on the synthesis of 6-keto-PGF1 alpha and TXB2 in vitro. But the ratio of metabolites of PGI2/TXA2 in vivo was increased in all ubiquinone groups. These results indicated that ubiquinone possesses protective effects on ischemic arrhythmia of conscious rats and the beneficial effects on myocardial ubiquinone content and PGI2/TXA2 seem to contribute to its myocardial protective action.