This study used HPLC to detect the concentration of morphine in cardiac blood of rats treated with intramuscular injection of therapeutic and toxic dose morphine. The results showed, in the group with therapeutic dose morphine, the cardiac blood concentration of morphine increased significantly within 96 h after death (P < 0.01), these increase of the cardiac blood morphine correlated well with time. In the group with toxic dose morphine, the cardiac blood concentration of morphine did not change obviously within 12 h after death, but increased gradually 24 h, 48 h, 96 h after death (P < 0.01), however, the morphine changes intensity is not more significant than the therapeutic dose. This study proved that the dose of morphine injected antemortem affects directly the postmortem cardiac blood morphine levels. It is analysed for the possible mechanism of the change.