Notes: Introduction: Action potential duration restitution (APDR) plays a role in initiation and maintenance of ventricular tachycardia (VT)/ventricular fibrillation (VF). We hypothesized that the steeply sloped APDR and its spatial heterogeneity contribute to VT/VF inducibility in patients with ventricular arrhythmia. Method and Results: After programmed ventricular stimulation (PVS) for evaluation of clinically documented VT, patients (n = 20, 15 male, age 52.5 ± 9.5 years) were divided into two groups: inducible sustained VT/VF (IVT, n = 10) and noninducible VT/VF (NVT, n = 10). Data were compared with the corresponding results obtained from normal controls (C, n = 10). Right ventricular (RV) monophasic action potential duration at 90% repolarization (APD90) and ventricular effective refractory period (VERP) in the right ventricular apex (RVA) and right ventricular outflow tract (RVOT) were determined. APDR was acquired by scanning diastole with premature ventricular beats during a pacing cycle length of 600 msec (S1-S2) in all patients and by rapid pacing at the cycle lengths that induced APD alternans in three patients. Maximal slopes (Smax) of the APDR curves and ΔAPD90 (APD90 at S2 400 ms – APD90 at the shortest S2) were measured. VERP and APD90 at each RV site did not differ among the three groups. Smax obtained by S1-S2 (1.6 ± 0.6) did not differ from Smax obtained by rapid pacing (1.2 ± 0.7), with a significant correlation noted between these values (r = 0.92, P 〈 0.01). The IVT group had a higher spatial dispersion of Smax (Smax at RVOT – Smax at RVA) compared to the C group (P 〈 0.05), with no difference between the NVT group and the IVT or C groups. The IVT group had a higher spatial dispersion of ΔAPD90 compared to the NVT and C groups (P 〈 0.01, respectively). Smax at the RVOT (2.7 ± 1.9) was steeper than that at the RVA (1.9 ± 1.2, P 〈 0.05). Inducibility of sustained VT/VF was greater at the RVOT (83.3%) than at the RVA (50.0%, P 〈 0.05). Conclusion: In patients with ventricular arrhythmia, VT/VF is highly inducible under conditions of greater spatial dispersion of ventricular refractoriness and APDR.