By use of a pump-oxygenator and total cardiopulmonary bypass bipolar ecgs have been recorded under direct vision from the bundle of His, right and left bundle branches and peripheral Purkinje system of the in situ dog heart. Electrical activity of these components of the specialized conducting system gives rise to distinct rapid ecg deflections clearly separable from activity in atria and ventricles. At 36[degree]C conduction velocity from the bundle of His to the right and left bundle branches is 1.0 to 1.5 m/sec, while in the false tendons of the right and left ventricles conduction velocity ranges from 3.0 to 4.0 m/sec.