Cross‐over infusions of quinidine (Q) and hydrvcjuinidine (HQ) ivere perfonned at 8‐day intenmls in 12 pentobarhital‐anesthetized dogs. Three increasing doses were administered each time and the mean plasma concentrations (mg/L) were 2.8, 3.7, and 4.9 with Q and 1.2, 1.9, and 2.8 with HQ. Four electrophysiological studies were perfonned at baseline and after each dose. Both drugs exhibited Class IA effects without qualitative differences: sinus automaticiti/ and nodal conduction parameters were increased only Zi'ith the highest doses of drugs; His Purkinje (HV) and intraventricular (QRS) conduction times were increased consistently but only slightly and, for example, mean values of HV increased from 28 to 33 msec after Q and 30 to 34 msec after HQ. The largest increases were seen for QT interval (227 to 294 msec after Q and 237 to 292 msec after HQ), ventricular effective refractory periods (158 to 182 msec after Q and 161 to 185 msec after HQ), and atrial refractory periods (124 to 178 msec after Q and 120 to 172 msec after HQ). For the later parameter, hydroquinidine appeared significantly more potent with a potenaj ratio relative to quinidine estimated at 1.9 and 2.9 respectively for effective and functional atrial refractory periods. If confirmed, this higher potency of hydroquinidine versus quinidine should be clinicalli/ considered, for example, during drug plasma levels monit