The purpose of this study was to test the hypothesis that abnormal intracellular calcium handling characterizes myocardial stunning. Isolated, isovolumic, buffer-perfused ferret hearts were loaded with the bioluminescent calcium indicator aequorin for simultaneous measurement of individual calcium transients and left ventricular pressure. After 15 minutes of global ischemia and 20 minutes of reperfusion, left ventricular developed pressure was significantly reduced (75±7 versus 93±6 mm Hg,p<0.05). During ischemia, [Ca2+], levels were significantly elevated compared with preischemic levels, both during systole (1.38±0.31 versus 0.88±0.2 μM,p<0.05) and end diastole (0.85±0.16 versus 0.38±0.13 μM,p<0.05). Early during reperfusion, [Ca2+]iwas also significantly elevated during systole (1.63±0.44 versus 0.88±0.20 μM,p<0.05) and end diastole (0.75±0.15 versus 0.38±0.13 μM,p<0.05). After 20 minutes of reperfusion, myocardial stunning occurred, but [Ca2+]iwas not significantly different from preischemic levels. Thus, myocardial stunning does not result from decreased levels of activator calcium. The force-pCa relation generated by the stunned hearts was shifted downward compared with that generated by the control hearts, consistent with a decrease in maximum calcium-activated force (Fmax). At steady state during tetanus, the decrease in Fmaxwas confirmed, but there was no significant difference in the slope of the force-pCa relation of the stunned hearts versus controls. Thus, we conclude that stunned myocardium is characterized by decreased Fmaxwithout desensitization of the myofilaments to [Ca2+]i. Elevations of [Ca2+]iduring ischemia and reperfusion precede myocardial stunning and may relate to its pathogenesis.