['Substantial progress has been made in our understanding of transmural activation across ventricular muscle through studies of excitation patterns and potential distributions. In contrast, repolarization sequences are poorly understood because of experimental difficulties in mapping action potential durations (APDs) using extracellular electrodes.', "Langendorff-perfused guinea pig hearts and isolated coronary-perfused left ventricular sheet preparations were stained with the voltage-sensitive dye RH-421 and optical APs were recorded with a photodiode array. Epicardial maps were constructed using a triangulation method applied to matrices of activation and repolarization times determined from (dF/dt)max and (d2F/dt2)max' respectively. Numerical simulations were carried out based on: (1) a modified Luo-Rudy model; (2) the three-dimensional architecture of ventricular fibers; and (3) the intrinsic spatial distribution of APDs. In ventricular sheets, epicardial stimulation elicited elliptical activation patterns with the major axis aligned with the longitudinal axis of epicardial fibers. When the pacing electrode was progressively inserted from epicardium to endocardium, the major axes rotated gradually, clockwise by 45 degrees, and the eccentricity decreased from 2 to 1.14. Repolarization showed a relatively uniform pattern, independent of pacing site, beginning at the apex and spreading to the base."] Are activation and repolarization patterns governed by different structural characteristics of ventricular myocardium : experimental study with voltage-sensitive dyes and numerical simulations?