Sudden cardiac death resulting from cardiac arrhythmia is a significant cause of mortality in Western Society. Effective electrical size (ratio of ventricular size to electrical activation wavelength) plays a significant role in governing reentrant arrhythmia dynamics. Due to similarities in effective size, the rabbit is suggested to be the most useful model for clinical investigations of fibrillatory arrhythmias(1,2). However, how well effective size of the rabbit, or other small mammalians, correlates to the human at slower pacing rates, as commonly seen during scar-related arrhythmias, and how it varies with frequency, is not well understood. In addition, electrophysiological changes in infarct-scar border-zone (BZ) cells(3,4) may cause crucial differences in effective electrical size and restitution between species which may be important in governing reentrant dynamics during infarction. Computational ionic ventricular cell models of human, rabbit, guinea pig and rat were used to investigate interspecies differences in action potential duration (APD), conduction velocity (CV) and wavelength restitution under S1S2 and dynamic restitution protocols. Effective size was calculated to observe how restitution effects combine to induce rate-dependent variations in effective size. Ionic changes were also implemented in rabbit and human models to simulate BZ remodelling and restitution protocols repeated. The human model displayed the steepest restitution curves for all metrics. The rat model displayed negative APD restitution, at single cell and tissue levels due to known limitations with this model. The rabbit model produced the most similar effective electrical size to the human across a range of activation rates (Fig 1), although differences in effective size became more apparent at higher frequencies due to the steeper APD and CV restitution properties of the human model. Similar results were observed between S1S2 and dynamic protocols. Incorporating electrophysiological changes to represent infarct BZ cells demonstrated further differences between rabbit and human models; the human showing more pronounced changes to restitution dynamics than the rabbit in BZ tissue relative to healthy myocardium, although less of an increase in absolute APD. Failure of the rabbit model to produce similar rate-dependent effects in effective size to the human in healthy or BZ tissue suggests potentially important interspecies differences in the initiation and anchoring of reentrant waves around structures, which are currently being investigated in preliminary simulations of reentry in 3D human and rabbit infarct models. This research highlights the need for further investigation into the utility of such models for informing the study of clinical scar-related arrhythmias.