The effect of potentials generated during depolarization of the left ventricle on epicardial unipolar electrograms recorded from the right ventricle was studied using the right ventricular isolation procedure. This surgical technique disrupts electrical conduction and prevents activation wavefronts from propagating between the ventricles. Following the procedure, the ventricles were paced asynchronously with the left ventricle paced 100 ms before the right ventricle to separate the electrogram into its local (due to depolarization of the right ventricle) and distant (due to depolarization of the left ventricle) components or with an interval of 20 ms or less between pacing the ventricles to mimic electrograms resulting from normal (synchronous) activation. Electrical activity in the left ventricle significantly affected the magnitude of the slope of the most rapid deflection and the timing of the maximum and minimum potentials of right ventricular unipolar electrograms. However, distant activity did not significantly alter the timing of the fastest 1 ms downstroke. No electrograms of distant components had negative slopes with magnitudes greater than 1.3 mV/ms, nor did any slopes of electrograms containing only local components have magnitudes less than 1.5 mV/ms. Simulated electrograms, calculated from the local and distant components, correlated well (r = 0.83 to 1.00, N = 48) with electrograms recorded during synchronous pacing. Thus, the magnitude, but not the timing of the fastest downstroke, which indicates the occurrence of local activation, was affected by the superposition of potentials due to distant and local electrical activity, and these potentials could be summed to produce electrograms which accurately reflected synchronous activity. © 1987 IEEE.