In 38 anesthetized and intubated dogs QRS features were estimated after incision of the interventricular septum. The left bundle branch system of the excised heart was stained with Lugol solution to visualize the extent and localization of the fascicular section. In 7 dogs the incision reached a part of the septal network; three of them showed incomplete LBBB pattern. In 13 dogs a major part of the septal network and the posterior/anterior fascile were transected. In 6 of them the pattern of incomplete LBBB was noted. A bipolar intracavitary L-R lead was recorded in 2 of these cases and revealed disappearance of the initial q wave in both. In 7 experiments the left bundle branch was completely transected with consecutive LBBB pattern. In 8 dogs the incision failed to reach the left bundle branch system and the ECG records did not show any form of LBBB pattern. The findings suggest that incomplete LBBB may be a form of left septal fascicular block.