Ca²⁺-induced Ca²⁺ release is a general mechanism that most cells use to amplify Ca²⁺ signals^,,,,. In heart cells, this mechanism is operated between voltage-gated L-type Ca²⁺ channels (LCCs) in the plasma membrane and Ca²⁺ release channels, commonly known as ryanodine receptors, in the sarcoplasmic reticulum^,,. The Ca²⁺ influx through LCCs traverses a cleft of roughly 12 nm formed by the cell surface and the sarcoplasmic reticulum membrane, and activates adjacent ryanodine receptors to release Ca²⁺ in the form of Ca²⁺ sparks. Here we determine the kinetics, fidelity and stoichiometry of coupling between LCCs and ryanodine receptors. We show that the local Ca²⁺ signal produced by a single opening of an LCC, named a ‘Ca²⁺ sparklet’, can trigger about 4–6 ryanodine receptors to generate a Ca²⁺ spark. The coupling between LCCs and ryanodine receptors is stochastic, as judged by the exponential distribution of the coupling latency. The fraction of sparklets that successfully triggers a spark is less than unity and declines in a use-dependent manner. This optical analysis of single-channel communication affords a powerful means for elucidating Ca²⁺-signalling mechanisms at the molecular level.