Hypothermia is a condition that leads to increased vulnerability of VF. Previous studies suggested that a short wavelength (WL), the product of action potential duration (APD) and conduction velocity (CV), is critical in arrhythmogenesis in mouse ventricles. To test the hypothesis that a dynamic APD restitution (APDR), in addition to short wavelengths, is important for the induction of VF, we performed optical mapping studies in 7 Langendorff-perfused mouse hearts. Low concentration cyto-D (5 關M/L) was used to inhibit motion. At 36째C, the APD averaged 81짹6 ms with pacing cycle length of 150 ms, the average CV was 58.1짹12.4 cm/sec, and WL 47.4짹9.2 mm. APDR determined in LV showed a maximum slope of the APDR of 0.78짹0.32. No spontaneous arrhythmia occurred. During hypothermia (28-33째C), the average AP increased to 123짹13 ms and the average CV decreased to 50.3짹9.3cm/sec, but the WL increased to 52.2짹10.6 mm (p=0.32). The maximum slope of APDR increased to 2.04짹0.95. Spontaneous onset of ventricular arrhythmias, including premature ventricular contractions (N=18), non-sustained VT (N=9) and VF (N=20) occurred in 4 of 7 hearts. At the onset of spontaneous ventricular arrhythmia, cells within a small region showed spontaneous oscillation of the action potential (AP), resembling early after depolarizations (EADs). In the meantime, the neighboring cells continued to repolarize without spontaneous oscillations. We conclude that steep slopes of APDR during hypothermia might have facilitated the induction and maintenance of VF through EADs and spatiotemporal heterogeneity of AP amplitudes. These heterogeneities then (resulted in reflection and phase-2 reentry,) leading to the spontaneous onset of ventricular arrhythmias.