['Transmural heterogeneity in ventricular repolarization demonstrated in vitro has been difficult to confirm in vivo. Whether this discrepancy reflects a physiological phenomenon or a methodological problem remains a vivid matter of debate despite a plethora of experimental work. Therefore, we have measured the relevant electrophysiological parameters first in vivo and repeated these in the same heart and at identical sites in vitro. Methodological issues were tackled by using both unipolar and bipolar recordings. Physiological issues were explored by measuring both local and functional electrophysiological parameters.', 'In 10 healthy dogs, 2 high-resolution needle electrodes were inserted into the left ventricle. Effective refractory periods (ERP) as well as activation recovery intervals (ARI) were determined at each electrode along both needles at basic cycle lengths (BCL) of 850 and 300 ms, respectively. After excision of the heart, ERP and ARI measurements were repeated in the arterially perfused wedge preparations.', 'First, we observed that ERPs and ARIs were significantly shorter in vivo than in vitro. Mean ERPs and ARIs of all muscle layers were relatively uniform throughout the ventricular wall in vivo. The transition from the in vivo to the in vitro preparation was associated with a significant albeit small increase of mean ARIs in the subendocardium, whereas interlayer differences in mean ERPs did not reach statistical significance as in vivo.'] Is there no transmural heterogeneity in an index of action potential duration in the canine left ventricle?