Cardiac transverse (t)-tubules play an essential role in ensuring synchronous calcium release from intracellular stores to enable contraction. In the adult heart of large mammals (including human), transverse (t)-tubules are present in the ventricles and atria (1). T-tubule loss in disease is associated with aberrant Ca2+ homeostasis and contractile dysfunction. Thus restoration of t-tubules is important therapeutically but the process by which t-tubules are formed is not understood. Previous studies using small mammals such as rats and rabbits have shown that the ventricular t-tubule network is absent or very sparse at birth and develops postnatally (2, 3), however there are no studies on the development of t-tubules in the atria. Newborn and adult sheep of approximately 18 months of age were euthanised with 200 mg/kg intravenous pentobarbitone and tissue was collected from the left atrial appendage and left ventricle. Tissue was fixed for 24 hours in 4% paraformaldehyde prior to embedding in paraffin wax blocks. After sectioning, tissue was dewaxed and rehydrated before heat-mediated antigen retrieval with sodium citrate buffer. T-tubule density was assessed using Wheat Germ Agglutinin (WGA) Alexa Fluor® 488 conjugate and confocal microscopy (1). Data is mean±SEM, statistical significance p<0.05 (t-test).Partially developed t-tubules were present at birth in sheep atrial and ventricular myocytes. The neonate had fewer t-tubules compared to the adult in both the atria and ventricle; the fractional area occupied by t-tubules within cells was 0.028±0.0048 vs. 0.064±0.0044, n=5 and p<0.001 in atrial myocytes, and 0.059±0.0053 vs. 0.10±0.0082, n=4 and p<0.01 in ventricular myocytes. Thus neonatal t-tubule density was lower than adult by 44±6.8% in the ventricle and 57±8.2% in the atria. The density of the atrial t-tubule network showed a positive correlation with cell width (p<0.001). T-tubules are present at birth in both the ventricle and atria of newborn sheep and develop postnatally to adulthood. Our data suggests that increasing cell width is associated with an increase in t-tubule density in the atria, which may be important in the dynamics of intracellular Ca2+ release.