The sinoatrial node (SAN) is the primary pacemaker of the mammalian heart and is where the heartbeat originates. Clinically, there has long been evidence that the SAN function differs according to gender; males have a slower resting and intrinsic heart rate compared to females1. It has also been shown that males have a longer corrected SAN recovery time and atrial refractory period than females2. The aim of this study was to investigate gene expression in the SAN of male and female rats in order to determine if changes in ion channel expression could underlie the observed functional differences. Ion channel gene (and related gene) expression was measured through quantitative PCR (qPCR) n=12 (n=6 from each group); GAPDH was used as the housekeeping gene. All results are given as mean ± SEM. Differences were evaluated by t test with Sigma Stat software. Differences were considered significant at the level of P<0.05. Intracellular action potential recordings were obtained using the sharp microelectrode technique (n=10, n=5 from each group). The amplitude of action potential was significantly less in the female SAN when compared with the male. Action potential duration (APD) at 15% and 75% was significantly shorter in the female SAN when compared with the male. The upstroke velocity (dV/dtmax) was significantly higher in the female SAN when compared to the male. Out of around 100 different transcripts investigated, only two ion channels were significantly different between the two sexes. The expression of the L-Type Ca2+ channel Cav1.3 mRNA was significantly higher in the female SAN compared to the male. Muscarinic K+ channel Kir3.1 mRNA was significantly higher in the female SAN when compared to male. Following histological staining with Masson’s trichrome of SAN preparations (n=4, n=2 from each group) to identify the location of the sinus node artery, we performed immunohistochemistry on adjacent sections to those used for histology. Our pilot immunohistochemistry data showed that Cav1.3 and Kir3.1 proteins were high in the female SAN (n=2) when compared with the male SAN (n=2). Both male and female SANs expressed HCN4 at mRNA and protein level but there was no difference between the two groups studied. All antibodies investigated in this study were tested for their specificity using the Western blot technique (n=4 male SAN). The research was conducted in accordance with the Guide for the Care and Use for Laboratory Animals in the UK. This study identified differences in key pacemaker ion channels between male and female SAN. Cav1.3 was recently shown to play an important role in the pacemaker function of the SAN3, therefore the higher intrinsic heart rate of the female SAN could be caused by the higher expression of Cav1.3. The differences identified in this study advance our understanding of gender differences in cardiac electrophysiology and may have implications for gender specific design of biological pacemakers.