Problem: Cardiac disease is a significant cause of morbidity and mortality in humans and dogs. Although rodent modelling systems allow for cost effective and rapid analysis the results obtained rarely directly translate into human disease systems. Larger animal models are required to study cardiac stem cell biology. Background: The dog is a species which develops naturally occurring cardiac disease; including clinical syndromes seen in humans. Moreover the pet dog is a close human companion, and therefore shares common environmental factors of disease. Hypothesis: Adult stem cells have been isolated from the heart in both mice and humans, which have been found to be multipotent and clonogenic. We propose that the dog also has a resident cardiac stem cell population, which can be isolated, characterized in terms of marker expression profiles and behaviour following cardiac directed differentiation allowing direct comparisons with human cardiac stem cell populations. Research: Using published techniques, atrial cardiac explants were taken from dogs post-mortem and cultured to isolate putative adult stem cells. These cells were characterized using an extended panel of markers, and compared to those expressed in human populations. The canine cells were subsequently placed into cardiac lineage differentiation protocols, and the expression profiles analyzed. Observations: Following isolation from cardiac explants, large round phase bright cells were harvested and placed into stem cell culture. These cells were able to survive successive passages in serum free media and formed large spherical cell clusters, termed ‘cardiospheres’. These cells were capable of clonal expansion under controlled culture conditions, demonstrating their ability for self-renewal. Characterization of marker profiles demonstrated c-kit, GATA 4 and flk-1 positive cells which were Nkx2.5 and cardiac lineage marker negative. Following cardiac directed differentiation marker profiles altered with upregulation of cardiac troponin T and Nkx2.5 and a down-regulation of c-kit and endothelial lineage markers. Conclusions: From our observations we have found a cell population that can be isolated from the canine heart and which behaves similarly to previously described human cardiac stem cells. Furthermore, we have demonstrated a change in marker expression consistent with cardiac directed differentiation. Further work will target isolating and characterising further adult stem cell populations from the dog, and exploring their differentiation potential. We conclude that canine adult stem cells are directly comparable to human stem cells, and are therefore an appropriate large animal modeling system for cardiac stem cell research. Discolsure: This work has been submitted for publication with The Veterinary Journal, and is currently under review.