The cardiac electrophysiologic and antiarrhythmic actions of two Class III ketone- and alcohol-containing spirobenzopyran piperidine analogs, L-702,958 and L-706,000 [MK-499], respectively, were assessed in vitro and in vivo. L-702,958 and L-706,000 [MK-499] selectively blocked the rapidly activating component of the delayed rectifier K+ current in guinea pig isolated ventricular myocytes (IC50 values, 14.6 and 43.9 nM, respectively), and prolonged effective refractory period in ferret isolated papillary muscles (EC25 values, 10.5 and 53.8 nM, respectively). In anesthetized dogs, L-702,958 and L-706,000 [MK-499] increased ventricular refractory periods (ED20 values, 3.3 and 9.2 micrograms/kg i.v., respectively) and concomitantly increased ECG QT interval and left ventricular+dP/dt. Cumulative i.v. administrations of up to 100 micrograms/kg of L-702,958 and 300 micrograms/kg L-706,000 [MK-499] in anesthetized dogs increased atrial and ventricular refractoriness and prolonged the ECG QT interval, but did not alter atrial, atrioventricular nodal, His-Purkinje or ventricular conduction indices. In anesthetized dogs studied chronically (9.2 +/- 1.1 days) after anterior myocardial infarction, the cumulative i.v. administrations of 100 micrograms/kg of L-702,958 and 300 of micrograms/kg L-706,000 [MK-499] suppressed the induction of ventricular tachyarrhythmia by programmed ventricular stimulation (suppression rates: 8 of 10, 80% and 9 of 11, 82%, respectively) and reduced the incidence of lethal ventricular arrhythmias (incidence of lethal ischemic arrhythmias: 4 of 10, 40% and 1 of 11 9%, respectively, compared to 34 of 40, 85%, in vehicle controls. L-702,958 and L-706,000 [MK-499] (cumulative 100 and 300 micrograms/kg i.v., respectively) did not facilitate the induction of arrhythmias by programmed ventricular stimulation in postinfarction dogs. After equivalently effective p.o. doses in conscious dogs, L-702,958 (10 micrograms/kg) and L-706,000 [MK-499] (30 micrograms/kg) increased ECG QT interval with long durations of action of approximately 9 and 14 hr, respectively. L-706,000 [MK-499] elicited a more consistent and sustained prolongation of the QT interval than L-702,958. These findings show that both L-702,958 and L-706,000 [MK-499] are potentially useful agents for the prevention of malignant ventricular arrhythmias in the setting of myocardial ischemic injury.