Local cardiac excitation is measured from either extracellular voltage (Ve) or optical transmembrane voltage (Vm) with fluorescent dye. We used a transparent electrode array and a laser scanner to test whether Ve and coepicentral optical Vm give equivalent excitation times during epicardial pacing and sinus beats. To help explain time differences, we estimated interrogation width/depth ratios for Ve and optical Vm with dipole and Monte Carlo models. Magnitudes of time differences between excitation measured with Ve and optical Vm during pacing were 1.9 +/- 1.9 ms (n= 1,112 recording pairs). Deeper interrogation for optical Vm vs Ve was indicated with sinus beats that contained a transmural propagation component. When pacing produced propagation along epicardial fibers, excitation time measured from optical Vm was later than excitation time measured with epicardial Ve. When pacing produced propagation across epicardial fibers, excitation time measured from optical Vm was earlier than that measured with epicardial Ve. These time differences could be attributed to deeper optical interrogation and fiber rotation with depth in ventricles.