Heart failure (HF) gives rise to remodelling of ion channel, Ca2+-handling, gap junction, fibrosis & apoptosis genes in the cardiac conduction system (CCS), which impairs pacemaker reserve & causes heart and bundle branch 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 study was conducted in accordance with the Guide for the Care & Use for Laboratory Animals & approved by the local animal research ethics committee. Data are presented as means±SEM & statistical differences assessed by Student’s t test, one way-ANOVA or two way ANOVA as appropriate. Differences were considered significant if P<0.05. After 6-7 weeks, heart/body weight ratio increased by 139% & fractional shortening fell by 50%. Internal ventricular systolic & diastolic dimensions increased by 58% & 22%, respectively. Recordings from conscious mice showed a 6% decrease in heart rate & 20% prolongation of QRS duration in the HF group. In anaesthetised mice, the PR interval was increased by 9% & QRS duration by 17% in the HF group. In isolated sinus node (SAN) preparations, cycle length was prolonged by 22% in the HF group. Superfusion of this preparation with 2 mM Cs (to block funny current, If) increased cycle length by 25% in the HF & 34% in the control mice. These data demonstrate dysfunction of the CCS consistent with changes in mRNA measured by qPCR, which showed significant downregulation of HCN4 (responsible for If) & Tbx18 (transcription factor). Expression of 384 miRs in the SAN was measured by microarray & compared to that in the left (LA) & right (RA) atrium. In the control group, 100 miRs showed significant differences in expression between the SAN & LA (47 were more abundant in SAN & 53 in LA), & 15 miRs showed significant differences in expression between the SAN & RA (11 were more abundant in SAN & 4 in RA). In HF there were 43 significant changes in miRs in the SAN. For example, miR-370-3p (using Ingenuity IPA software, predicted to affect HCN4) was upregulated by 81% in the SAN & 57% in the LA. To validate the effect of this microRNA on the SAN, miR-370-3p mimic was injected into SAN tissue preparations & maintained in culture medium for ~48 hours. miR-370-3p resulted in significant bradycardia 5 hours after injection (heart rate was 289 ± 5 bpm in SAN preparation injected with scrambled miR & 231 ± 24 bpm injected with miR-370-3p) & this effect continued & after 20 hours the heart rate was 284 ± 20 bpm in the control SAN preparations & 193 ± 18 bpm in the SAN preparations injected with miR-370-3p mimic. These data reveal that increased expression of endogenous miR-370-3p contributes to bradycardia associated with HF & suggest that it might be targeted therapeutically to restore function.