When guinea pig cardiac tissue is stimulated, the rising phase of the action potential is influenced by the shape of the concomitant local surface electrogram. This, in turn, depends on the direction of the wave of excitation passing the recording microelectrode. 2. A symmetrical biphasic surface electrogram yields sigmoid upstrokes with masimum upstroke velocity (Vmax) lower than when it is either zero or uniphasic and negative. 3. When the main direction of the depolarization wave in guinea pig ventricular myocardium is receding from the recording microelectrode, giving rise to negative uniphasic extracellular potentials, a notch might distort the registration of the rising phase of the action potential. 4. When the main direction of the depolarization wave is approaching the recording microelectrode, giving rise to positive uniphasic extracellular potentials, Vmax is transferred to less negative membrane potentials. 5. The extracellular surface electrogram influences the overshoot of the action potentials of the normal atrial and depressed (D-600) ventricular myocardium and is changed, depending on how the wave of excitation passes the recording microelectrode.
