Significance To directly address in vivo significance of the altered Ca _V 1.2 channel property arising from alternative splicing, we generated Ca _V 1.2 exon 33-specific knockout (exon 33 ^−/− ) mice. Here, we showed that the exclusion of alternative exon 33 altered Ca _V 1.2 biophysical property, leading to greater I _Ca density. This increase in current density induced prolongation of ventricular cardiomyocyte action potential duration, and the cardiomyocytes exhibited increased early afterdepolarizations and autonomous action potentials—hallmarks of arrhythmias. In vivo, exon 33 ^−/− mice had increased occurrences of premature ventricular contractions, tachycardia, and lengthened QT interval. As such, exclusion of exon 33 of the Ca _V 1.2 channel is proarrhythmogenic. Although failing human hearts had greater inclusion of exon 33, it is unclear whether the inclusion is compensatory, neutral, or damaging.