Heart failure (HF) gives rise to remodelling of ion channel, Ca2+-handling, gap junction, fibrosis and apoptosis genes in the cardiac conduction system (CCS), which impairs pacemaker reserve and causes heart block. MicroRNAs (miRs) are small non-coding RNAs which regulate gene expression. We have tested the hypothesis that they are responsible for this remodelling by using mice in which HF was produced by transverse aortic constriction. The animal studies performed were in accordance with the UK Home Office and institutional guidelines. After 6-7 weeks, heart/body weight ratio increased by 139% and fractional shortening fell by 50%. Left ventricular mass, and internal systolic and diastolic dimensions increased by 85%, 58% and 22%, respectively. Recordings from conscious mice showed 6% decrease in heart rate and 20% prolongation of QRS duration in HF group. In anaesthetised mice (2% isoflurane), PR interval was increased by 9% and QRS duration by 17% in the HF group. In isolated sinoatrial node (SAN) preparations cycle length was prolonged by 22% in the HF group. Superfusion of this preparation with 2 mM Cs (to block the funny current, If) increased cycle length by 25% in the HF group and 34% in the control group. These data demonstrate dysfunction of the CCS consistent with changes in mRNA measured by qPCR, which showed significant downregulation of HCN4 (responsible for If) and Tbx18 and Mef2c (transcription factors), and upregulation of Col1α1 (collagen type 1 α 1). Expression of 384 miRs in the SAN was measured by microarray and compared to that in the left (LA) and right (RA) atrium. In the control group, 100 miRs showed significant differences in expression between the SAN and LA (47 were more abundant in SAN and 53 in LA), and 15 miRs showed significant differences in expression between the SAN and RA (11 were more abundant in SAN and 4 in RA). In HF there were 43 significant changes in miRs in the SAN. For example, miR-139-3p (using Ingenuity IPA software, predicted to affect HCN1, HCN4, SERCA2, RYR2, ERK2, Tbx3, Tbx18, REST, AP1 and Mef2c) was upregulated by 1000% in the SAN. miR-21 (predicted to affect Col1α1, ERK2, Nkx2-5, and Mef2c) was upregulated by 149% in the SAN. miR-202-3p (predicted to affect HCN1, HCN2, HCN4, SERCA2, Cx40, Cx43, collagen 3 α 1, Tbx2, Tbx3, Tbx5, Tbx18, HAND1, HAND2 and Mef2c) was dowregulated by 97% in the SAN. miR-130b, miR-19a and miR-301a/b were predicted to affect ERK2 responsible for cardiac hypertrophy with pressure overload (downregulated by 190, 100, 85 and 80%, respectively). These altered miRs are predicted to target ion channel, Ca2+-handling, gap junction, fibrosis, apoptosis and regulatory genes. In conclusion we have found both up- and downregulation of miRs in HF, and we suggest that this could be responsible for the adverse remodelling of the CCS.