Spontaneous local increases in the concentration of intracellular calcium ( (Ca{dollar}\sp{lcub}2+{rcub}\rbrack \rm\sb{lcub}i{rcub}{dollar}), called "calcium sparks", were detected in quiescent rat heart cells with a confocal laser scanning microscope and the fluorescent calcium indicator fluo-3. A calcium spark is associated with an elevation of (Ca{dollar}\sp{lcub}2+{rcub}\rbrack \rm\sb{lcub}i{rcub}{dollar} by {dollar}\sim{dollar}200 nM within a volume of {dollar}\sim{dollar}10 fl, and to decline with an half time of {dollar}\sim{dollar}20 msec. Estimates of calcium flux associated with the local increase in (Ca{dollar}\sp{lcub}2+{rcub}\rbrack \rm\sb{lcub}i{rcub}{dollar} suggest that calcium sparks arise from the spontaneous openings of single or a few sarcoplasmic reticulum (SR) calcium-release channels (also known as ryanodine receptors, RyRs) acting in concert, a finding supported by ryanodine modification of spark kinetics. Thus calcium sparks represent the functional elementary events of the SR release of calcium. By measuring the occurrence of calcium, the in vivo open probability of RyR/channels is shown to be around 0.0001 S{dollar}\sp{lcub}-1{rcub}{dollar} at resting (Ca{dollar}\sp{lcub}2+{rcub}\rbrack \rm\sb{lcub}i{rcub}{dollar}. It is generally agreed that during cardiac excitation-contraction (EC) coupling, calcium release from the SR is triggered by the sarcolemmal calcium current (I{dollar}\rm\sb{lcub}Ca{rcub}{dollar}) via the calcium-induced calcium release mechanism. However, it is unclear how a mechanism with intrinsic positive feedback can provide graded responses. This work reveals, for the first time, that at the microscopic level, EC coupling takes the form of I{dollar}\rm\sb{lcub}Ca{rcub}{dollar}-evoked calcium sparks. Direct visualization of evoked calcium sparks was possible when I{dollar}\rm\sb{lcub}Ca{rcub}{dollar} was reduced by calcium channel antagonists D600 or cadmium, or during small ramp depolarization under whole-cell voltage-clamp conditions. These evoked calcium sparks resemble spontaneous calcium sparks observed at rest, in amplitude and in spatio-temporal properties. The activation of calcium sparks is controlled by local I{dollar}\rm\sb{lcub}Ca{rcub}{dollar} in a stochastic manner. Once activated, calcium release from the SR during a calcium spark is essentially independent of the triggering calcium influx and does not activate neighboring SR release sites. These novel findings are used to develop a new mechanistic model for cardiac EC coupling in which the graded amplification of the triggering I{dollar}\rm\sb{lcub}Ca{rcub}{dollar} by the SR can be explained by altering the extent of spatial and temporal summation of the elementary release events.