Filtered bipolar catheter electodes, I mm apart, were used to pace and record from the high right atrium, right ventricular apex, right ventricular outflow tract, right ventricular inflow tract, middle cardiac vein, great cardiac vein, and endocardium of the left ventricular septal surface. Right ventricular apex to middle cardiac vein and the middle cardiac vein to right ventricular apex conduction intervals gave a rough estimate of anteroposterior and posteroanterior "transseptal plus free left ventricular wall" conduction times, respectively. On the other hand, the right ventricular apex to left ventricular septal surface and left ventricular septal surface to right ventricular apex intervals represented pure "transseptal" conduction times, since both sets of electrodes were in contact with the respective septal surface. During stimulation of the intermediately located right ventricular inflow tract propagation to the right ventricular apex and right ventricular outflow tract was longer than between these two sites. Moreover, conduction was almost as delayed to the right ventricular apex and right ventricular outflow tract as it was to the left ventricular septal surface. These findings were attributed to the peculiar electrophysiological behaviour of the right ventricular inflow tract muscle. Pacing from different segments of the great cardiac vein produced QRS morphologies and arrival of excitation patterns consistent with the relation between the anatomical location of this structure and the recording electrodes. However, from this study no inferences could be drawn regarding the conduction velocity or specific conduction pathways used by the stimulus in its journey from stimulating to recording areas.