The electrophysiological effects of ajmaline have been investigated using the intracardiac electrograms and the extrastimulus method. The drug was infused intravenously into 21 patients at the dose of 1 mg/Kg. Ajmaline induces:--variable effect on the sinusal cycle length--variable effect on the corrected sinus node recovery time--increase of the atrial effective refractory period (P less than 0.001)--in AV node slowing of conduction (P less than 0.001), a slight prolongation of PRE (P less 0.01) and more marked one of the FRP (P less than 0.001). The His-Purkinje (HV interval) and intraventicular (QRS interval) conduction times are prolonged (P less than 0.001), as well as the RRF of His-Purkinje system. The maxium increase of AH,HV and QRS intervals appears 3 minutes after drug administration; at the 5th minute these intervals have already slightly decreased, then they decrease almost quickly till the 15th minute and more slowly till the 30th minute. The absolute prolongation of the HV and QRS intervals is more marked in the patients in which these intervals are more prolonged during the control study, while the percentage increase is almost equal in the patients with normal HV and QRS interval and in those with prolonged HV and QRS intervals. The electrophysiological properties of the drug are discussed on the basis of the obtained results.