Many cardiovascular variables, such as heart rate, cardiac contractility, stroke volume, ECG waveforms (RR, PR, QRS and QT intervals) and blood pressure, exhibit robust day-night (circadian) rhythms. Many cardiac arrhythmias also display a circadian rhythm and occur preferentially at a particular time of day or night. For example, ventricular fibrillation and sudden cardiac death demonstrate a prominent circadian rhythm, being more common in the morning on waking (Black et al., 2019). A local circadian clock in the heart may be ultimately responsible for these rhythms. Ca2+/calmodulin-dependent protein kinase II (CaMKII) plays a major role in Ca2+ homeostasis in the heart. It also plays an essential role in circadian rhythms by facilitating the heterodimerisation of the circadian clock genes, Clock and Bmal1. CaMKII overexpression has been associated with ventricular arrhythmias, and knockdown of CaMKII prevents ventricular tachycardia. In an animal model, this study aimed to determine whether CaMKII is responsible for the ventricular arrhythmias seen during the awake period. Experiments were conducted on ex vivo Langendorff-perfused hearts from cardiac-specific CaMKII deficient mice (expressing AC3-I inhibitory peptide) and their littermate controls (not expressing the peptide) (Wu et al., 2009). The hearts were dissected at either zeitgeber time (ZT) 0 (start of the sleep period) or at ZT 12 (start of the awake period). A S1-S2 pacing protocol was used to test the vulnerability to ventricular arrhythmias. In control mice, ventricular arrhythmias were inducible at the start of awake period in 4 out of 9 hearts, but not at the start of the sleep period (0 out of 8 hearts; P=0.02, Chi-squared test), which is consistent with the morning peak in ventricular arrhythmias in the human. In contrast, in the CaMKII deficient mice, ventricular arrhythmias were not observed at the start of the awake or sleep periods (0 out of 5 hearts at ZT 0 and ZT 12). Expression of genes was measured by quantitative PCR. In CaMKII deficient mice, the expression of the circadian genes, Bmal1 and Clock, in the left ventricle was increased at ZT 0 (Bmal1, P<0.001; Clock, P<0.0005; two-way ANOVA). Additionally, in the left ventricle of CaMKII deficient mice, there was a reduction in the intracellular Ca2+-handling molecules Pln and Ryr2 at ZT 12 (P<0.0001; two-way ANOVA). In summary, this study has shown a circadian rhythm in ventricular arrhythmias in the mouse that is abolished by inhibition of CaMKII. This may be linked to effects on the local circadian clock in the heart as well as on intracellular Ca2+-handling.