Abstract We studied three groups of individuals by means of spatial-velocity electrocardiograms and thallium-201 myocardial imaging to figure out the electrophysiological explanation of the S I S II S III electrocardiographic morphology. We studied twelve healthy individuals without S I S II S III, seven healthy individuals with S I S II S III and fifteen patients with chronic obstructive pulmonary disease with S I S II S III. The average values of the QRS-E and QRS-F intervals were higher in the second ( P < 0.05 and P < 0.005) and third groups ( P < 0.01 and P < 0.001) than in the first. One patient of the second group and thirteen of the third showed right ventricular enlargement. The slowing down of the right ventricular conduction explained the S I S II S III morphology in normal individuals in more than half the cases. In patients with chronic obstructive pulmonary disease with S I S II S III the conduction delay plays an important part in the electrogenesis of the right ventricular enlargement electrocardiographic morphology. We think that these observations can give further data about the electrophysiologic mechanism of the S I S II S III morphology.