microRNAs (miRNAs) are post-transcriptional regulators of gene expression that have been shown to play an important role in the establishment of cellular programs. The heart develops from two small patches of cells in the mesoderm, the heart fields, which originate the different cardiac cell types, including cardiomyocytes, vascular smooth muscle and endothelial cells. These progenitors proliferate and differentiate to establish a highly connected three-dimensional structure, involving a robust succession of gene expression programs strongly influenced by miRNAs.To test the role of miRNA in the development of cardiac conduction system, sinoatrial node (SAN) from embryonic day E 12.5 and E18.5, and from 1-day-old and 10-day-old mice were flash frozen in cold isopentane and stored at −80°C. The study was conducted in accordance with the Guide for the Care and Use for Laboratory Animals. Tissues collected from 2 hearts were combined to increase the yield of total RNA from each region (n=18 mice). Single-stranded cDNA was synthesised from 100 ng total RNA. Using qPCR, I investigated the changes in ion channels and transcription factors at 4 different time points: at E12.5 and E18.5 and at 1-day and 10-days postnatal. Data are presented as means±SEM and statistical differences assessed by Student’s t test, one way-ANOVA or as appropriate. Differences were considered significant if P<0.05. n corresponds to the number of animals.My results showed that Tbx18 (which is essential for the formation of SAN) was significantly higher at E12.5 than at other time points of development. In contrast, Tbx3 did not show significant differences at any time point. In the SAN, HCN4 mRNA was significantly higher at E18.5 than at E12.5 and day 1 postnatal. HCN1 was significantly higher at day 10 postnatal than at E12.5, E18.5 and day 1 postnatal. SERCA2 and RYR2 showed significant upregulation immediately after birth. The expression of Cx43 did not show significant changes at different time points. Nkx2.5 significantly higher at E 12.5 than other time points. The same total RNA (enriched for miRNAs) was reverse transcribed for miRNAs and individual miRNA primers were used to investigate the pattern of expression of miRNAs in the SAN. miR-133 a,b and miR-29a were significantly higher at day 10 postnatal than at E12.5 and E18.5 and day 1 postnatal. miR-1 was significantly more abundant in the SAN in postnatal mice when compared to E12.5 and day 1 postnatal. miR-328 showed gradual upregulation from E12.5 to day 10 postnatal. miR-20a was significantly higher at E18.5 when compared with E12.5 and postnatal day 1. miR-126 significantly higher at 10 day postnatal when compared to E12.5 and E18.5 and day 1 postnatal. In conclusion, miRNAs play an important role in the embryonic development of SAN.