In the heart, oxidative stress caused by exogenous H₂O₂ has been shown to induce early afterdepolarizations (EADs) and triggered activity by impairing Na current (I_Na) inactivation. Because H₂O₂ activates Ca²⁺/calmodulin kinase (CaMK)II, which also impairs I_Na inactivation and promotes EADs, we hypothesized that CaMKII activation may be an important factor in EADs caused by oxidative stress. Using the patch-clamp and intracellular Ca (Ca_i) imaging in Fluo-4 AM–loaded rabbit ventricular myocytes, we found that exposure to H₂O₂ (0.2 to 1 mmol/L) for 5 to 15 minutes consistently induced EADs that were suppressed by the I_Na blocker tetrodotoxin (10 μmol/L), as well as the I_Ca,L blocker nifedipine. H₂O₂ enhanced both peak and late I_Ca,L, consistent with CaMKII-mediated facilitation. By prolonging the action potential plateau and increasing Ca influx via I_Ca,L, H₂O₂-induced EADs were also frequently followed by DADs in response to spontaneous (ie, non–I_Ca,L-gated) sarcoplasmic reticulum Ca release after repolarization. The CaMKII inhibitor KN-93 (1 μmol/L; n=4), but not its inactive analog KN-92 (1 μmol/L, n=5), prevented H₂O₂-induced EADs and DADs, and the selective CaMKII peptide inhibitor AIP (autocamtide-2–related inhibitory peptide) (2 μmol/L) significantly delayed their onset. In conclusion, H₂O₂-induced afterdepolarizations depend on both impaired I_Na inactivation to reduce repolarization reserve and enhancement of I_Ca,L to reverse repolarization, which are both facilitated by CaMKII activation. Our observations support a link between increased oxidative stress, CaMKII activation, and afterdepolarizations as triggers of lethal ventricular arrhythmias in diseased hearts.