Cardiac ryanodine receptor (RyR2) mutations are associated with autosomal dominant catecholaminergic polymorphic ventricular tachycardia, suggesting that alterations in Ca ²⁺ handling underlie this disease. Here we analyze the underlying Ca ²⁺ release defect that leads to arrhythmia in cardiomyocytes isolated from heterozygous knock-in mice carrying the RyR2 ^R4496C mutation. RyR2 ^{R4496C−/−} littermates (wild type) were used as controls. [Ca ²⁺ ] _i transients were obtained by field stimulation in fluo-3–loaded cardiomyocytes and viewed using confocal microscopy. In our basal recording conditions (2-Hz stimulation rate), [Ca ²⁺ ] _i transients and sarcoplasmic reticulum Ca ²⁺ load were similar in wild-type and RyR2 ^R4496C cells. However, paced RyR2 ^R4496C ventricular myocytes presented abnormal Ca ²⁺ release during the diastolic period, viewed as Ca ²⁺ waves, consistent with the occurrence of delayed afterdepolarizations. The occurrence of this abnormal Ca ²⁺ release was enhanced at faster stimulation rates and by β-adrenergic stimulation, which also induced triggered activity. Spontaneous Ca ²⁺ sparks were more frequent in RyR2 ^R4496C myocytes, indicating increased RyR2 ^R4496C activity. When permeabilized cells were exposed to different cytosolic [Ca ²⁺ ] _i , RyR2 ^R4496C showed a dramatic increase in Ca ²⁺ sensitivity. Isoproterenol increased [Ca ²⁺ ] _i transient amplitude and Ca ²⁺ spark frequency to the same extent in wild-type and RyR2 ^R4496C cells, indicating that the β-adrenergic sensitivity of RyR2 ^R4496C cells remained unaltered. This effect was independent of protein expression variations because no difference was found in the total or phosphorylated RyR2 expression levels. In conclusion, the arrhythmogenic potential of the RyR2 ^R4496C mutation is attributable to the increased Ca ²⁺ sensitivity of RyR2 ^R4496C , which induces diastolic Ca ²⁺ release and lowers the threshold for triggered activity.