A bidomain reaction-diffusion model of the human heart was developed and potentials resulting from normal depolarization and repolarization were compared with results from a compatible monodomain model. Comparisons were made for an empty isolated heart and for a heart with fluid-filled ventricles. Both sinus rhythm and ectopic activation were simulated. The model took 2 days on 32 processors to simulate a complete cardiac cycle. Differences between monodomain and bidomain results were very small, even for the extracellular potentials which, for the monodomain model, were computed with a high-resolution forward model. Electrograms computed with monodomain and bidomain models were visually indistinguishable. We conclude that, in the absence of applied currents, propagating action potentials on the scale of a human heart can be studied with a monodomain model