INTRODUCTION: A three-dimensional bidomain model of a block section of both the right and left ventricular walls that included rotational anisotropy and fiber curvature was used to investigate potential distributions generated during paced activation mapping. Unlike previous large-scale tissue models, the extracellular stimulus was included. METHODS AND RESULTS: The model was used to test the hypothesis that information about the underlying tissue structure (surface fiber angle gradients, amount of fiber rotation per unit depth, and anisotropy) can be extracted from surface potential distributions during stimulation. Results from distributions during stimulation were compared to those obtained using the distributions during activation. To better correlate results to possible experimental measurements, the analysis was performed using a 21 X 21 grid of "electrode" sites, each separated by 1 mm. Fiber orientation was estimated from the surface data by: (1) curve-fitting the elliptical shape of the epicardial potential distribution during stimulation; (2) identifying the location of the potential maxima leading the wavefront during early activation; and (3) for epicardial stimuli, curve-fitting the elliptical shape of the activation isochrones. Results show that surface potential distributions from the stimulus can be used to estimate fiber orientation; however, the accuracy of the reconstruction is highly dependent on the amount of fiber rotation per unit depth. CONCLUSIONS: Extracellular potential data during and after stimulation is shown to reflect the organization of myocardial fibers and, as such, could be used to characterize the three-dimensional anisotropic electrical properties in situ.