Although Ca2+waves in cardiac myocytes are regarded as arrhythmogenic substrates, their properties in the heart in situ are poorly understood. On the hypothesis that Ca2+waves in the heart behave diversely and some of them influence the cardiac function, we analyzed their incidence, propagation velocity, and intercellular propagation at the subepicardial myocardium of fluo 3–loaded rat whole hearts using real-time laser scanning confocal microscopy. We classified Ca2+waves into 3 types. In intact regions showing homogeneous Ca2+transients under sinus rhythm (2 mmol/L [Ca2+]o), Ca2+waves did not occur. Under quiescence, the waves occurred sporadically (3.8 waves · min−1· cell−1), with a velocity of 84 &mgr;m/s, a decline half-time (t1/2) of 0.16 seconds, and rare intercellular propagation (propagation ratio <0.06) (sporadic wave). In contrast, in presumably Ca2+-overloaded regions showing higher fluorescent intensity (113% versus the intact regions), Ca2+waves occurred at 28 waves · min−1· cell−1under quiescence with a higher velocity (116 &mgr;m/s), longer decline time (t1/2=0.41 second), and occasional intercellular propagation (propagation ratio=0.23) (Ca2+-overloaded wave). In regions with much higher fluorescent intensity (124% versus the intact region), Ca2+waves occurred with a high incidence (133 waves · min−1· cell−1) and little intercellular propagation (agonal wave). We conclude that the spatiotemporal properties of Ca2+waves in the heart are diverse and modulated by the Ca2+-loading state. The sporadic waves would not affect cardiac function, but prevalent Ca2+-overloaded and agonal waves may induce contractile failure and arrhythmias.