Atrial fibrillation (AF) is the most common cardiac arrhythmia and the primary cause of stroke. Incidence of atrial arrhythmias show circadian variation which could be linked to the disruption of cardiac clock genes rhythmicity. Per 1 and Per2 clock genes have been reported in mouse atrial explant1. AF is associated with raised intracellular Ca2+ concentration ([Ca2+]i) partially caused by the activation of persistent sodium current2. The latter is activated by anemonia sulcata toxin II (ATXII). This study aims to: 1) determine Per2 and Bmal1 gene expression rhythmicity in culture atrial myocytes and 2) assess the effects of ATXII on Per2 and Bmal1 gene expression rhythmicity. Atrial cultured cells (HL-1-6)3 were grown in Claycomb medium (10% FBS, 0.1 mM norepinephrine and 2 mM L-glutamine). Mouse embryonic fibroblasts (MEF; used as a positive control) were maintained in DMEM with 10% FBS. Cells were incubated with medium containing 50% FBS for 2hrs to synchronise Per2 and Bmal1 gene expression to a zero clock time, defined as Zeitgeber time (ZT). Atrial arrhythmia was induced by incubating cells with ATXII (1nM) throughout the experiment. Cells were then collected at timed intervals for RT-PCR. Per2, Bmal1 and GAPDH mRNA levels (defined by cycle threshold- CT) were measured. ΔCT is the difference between clock genes and their GAPDH CT values. In addition, cells were transfected with Bmal1::luciferase probe to determine bioluminescence rhythmicity using a Lumicycler. Two-Way ANOVA was initially used to test significance between groups followed by Bonferroni post hoc comparison. Values are mean±SEM. Differences were deemed significant at p<0.05. HL-1-6 cells transfected with Bmal1::luciferase exhibited 24 hrs circadian rhythm pattern (26.54±3.0 hrs; n=3) similar to MEFs (25.69±2.47 hrs; n=4). In HL-1-6 cells, Per2 and Bmal1 mRNA exhibited anti-phasic rhythmic expression similar to MEFs. Per2 mRNA levels and rhythmicity were maintained in HL-1-6 cells in control and ATXII. However, ATX II caused a 12 hrs phase shift in the ΔCT Per2 peak gene expression when compared with control. At ZT 18 Per2 gene expression was significantly higher in ATXII treated vs control cells (Control ΔCT 7.89±0.16 vs ATX ΔCT 9.13±0.26; n=3; p<0.01). Similarly, at ZT 12 Bmal1 gene expression was significantly higher in ATXII treated versus control cells (Control ΔCT 7.57±0.21 vs ATX ΔCT 8.70±0.34; n=3; p<0.01). This study showed that in cultured atrial myocytes, Per2 and Bmal1 genes are endogenously expressed and exhibit 24 hrs circadian rhythm under control conditions. However, with ATXII, a significant increase in Per2 and Bmal1 gene expression levels were observed at a given ZT. Also, ATXII caused a 12 hrs phase shift in Per2 gene expression. In conclusion, this study provides a novel correlation between ATXII induced atrial arrhythmias and Per2 and Bmal1 gene rhythmicity patterns, indicating a role for clock genes in atrial arrhythmogenesis.