The aim of this work was to demonstrate the correspondence between a macroscopic 3D computer model of electrophysiology (i.e., the Aliev-Panfilov model) parametrized with MR data and experimental characterization of action potential propagation in large porcine hearts, ex vivo, using optical methods (based on voltage-sensitive fluorescence). A secondary goal was to use one of these studies to demonstrate an optimized method for regional adjustment of critical model parameters (i.e., adjustment of the local conductivity from the isochronal maps obtained via optical images). There was good agreement between model behaviour and experiment using fusion of optical and MR data, and model parameters from previous work in the literature. Specifically, qualitative comparison between computed and measured activation maps gave good results. Adjustment of the conductivity parameter within 26 regions fitting data from the current experiments in one heart reduced absolute error in local depolarization times by a factor of 3 (i.e. from 30 to 10 ms).