A system of digital measuring of the cardiac action potential was constructed in order to measure quickly and accurately the resting potential or maximum diastolic potential, overshoot potential, amplitude, time for 50% repolarization and time for 90% repolarization. This system consists of a dual-beam oscilloscope, a digital voltage meter, a digital interval meter and a simple circuit constructed by operational amplifiers. The measurement of the action potential was achieved by tracing only the resting potential or the maximum diastolic potential and the crest of the action potential on the oscilloscope using another beam. The time for measurement is usually 10-15 sec and errors in measurement were considered to be negligible. This system is considered to be useful for digital measurement of the action potential of various cardiac tissues. This system was applied for observation of effects of bufetolol, an adrenergic beta-receptor blocking drug on the action potential of the dog Purkinje fibres. Bufetolol (10(-5) M) caused a slight depolarization of the maximum diastolic potential, decreased overshoot potential, amplitude and maximum rate of rise, and shortened the time for 50% repolarization of the action potential. Bufetolol (10(-4) M) additionally prolonged the time for 90% repolarization.