A Langendorff perfused rat heart preparation was designed to process dose-response effects of cardioactive drugs on rate, coronary flow, and supraaortic differential pressure (delta P; an index of cardiac performance). In this preparation, delta 9- -tetrahydrocannabinol (THC) 2 X 10(-6) M to 10(-5) M induces in the isolated perfused rat heart a biphasic increase in rate (maximal at 8 X 10(-6) M). Tachycardia is associated with decreases in (delta P) and no change or decreased coronary flow. Cardiac toxicity is observed with 3 X 10(-5) M. Cannabidiol (CBD) at concentrations of 9 X 10(-6) M to 10(-4) M has limited effect on rate while increasing delta P and coronary flow. Cannabinol (CBN) 8 X 10(-6) M to 3 X 10(-4) M depresses rate and delta P while coronary flow remains constant. Simultaneous equimolar administration of THC with CBD antagonizes or mitigates the cardiac effects of THC on rate, delta P, and coronary flow.