AIM To study the effects of the 50 Hz-filter circuit in a microelectrode amplifier on cardiac action potential waveform and parameters. METHODS Cardiac action potential signals were fed into a microcomputer through a glass microelectrode, a microelectrode amplifier, a differentiator and A/D converter. The cardiac action potential signals were recorded and analyzed with 50 Hz-filter circuit and without it, and the frequency spectrum in the signals was analyzed with the fast Fourier transformation. RESULTS When the 50 Hz-filter circuit was used, the phase 0 of the potential waveform was seriously distorted and prolonged. The maximal rate of depolarization at the phase 0 was cut down, while the other parameters were not effected. CONCLUSION There has already been much 50 Hz element in the action potential waveform. During amplifying the cardiac action potential signal, the 50 Hz-filter circuit should not be turned on. Otherwise, the experiment results will be effected.