A mouse cell staining with Di-8-ANEPPS is applied voltage clamp pulses and immersed in Na ⁺ -containing solution. The Di-8-ANEPPS signal, which largely reflects t system voltage, has an asymmetrical positive component upon application of depolarizing pulses. This is interpreted as reflecting a propagating action potential, escaping from the voltage-clamped plasma membrane. Its temporal properties are not unlike those of the “center” signal. Notably, the peak of the signal occurrs 0.3 ms after the leading edge of the depolarizing pulse.