DPI201-106 is a new positive inotropic agent. The cardiac electrophysiology of 16 patients was studied before and during DPI 201-106 administration (loading dose of intravenous DPI 201-106, 1.8 mg kg-1 h-1 administered over 10 min, followed by a maintenance dose of 0.2 mg kg-1 h-1). DPI 201-106 had no effect on the sinus node. The AH interval during fixed-rate atrial pacing became prolonged during DPI 201-106 infusion. There was a significant prolongation of the QT interval [QT (corrected), 417 +/- 22 to 502 +/- 35 ms, P less than 0.05; QT (atrial pacing at 600 ms), 374 +/- 17 to 419 +/- 23 ms, P less than 0.05; QT (ventricular pacing at 600 ms), 409 +/- 37 to 449 +/- 30 ms, P less than 0.05]. The ventricular effective refractory period significantly prolonged during DPI 201-106 administration (242 +/- 21 to 287 +/- 56 ms, P less than 0.05), but the supernormal-period duration decreased. The atrial effective refractory period was shortened in four patients and prolonged in one (261 +/- 67 to 240 +/- 53 ms, NS). The corrected atrial repolarization time (PTac) shortened significantly during DPI 210-106 infusion (479 +/- 26 to 445 +/- 22 ms at 20 min of the maintenance dose, P less than 0.05). Atrial fibrillation was initiated in five patients during DPI infusion, but no ventricular arrhythmia was provoked. These findings suggest that DPI 201-106 has novel differential electrophysiological effects on atria and ventricles.