One hypothesis for arrhythmogenesis is that a spontaneous ectopic excitation occurring during the vulnerable window of a propagating wave can initiate reentry. This has been demonstrated experimentally and computationally with artificial initiation methods (Starmer et al. 1993; Clayton & Holden, 2002) but spontaneous onset of reentry is rarely observed. The ectopic events that leads to early afterdepolarizations (EADs) in single cells, unidirectional propagations in 1-D fibre, reentrant spiral waves in 2-D tissue and rotors and scroll rings in 3-D slabs, could be the result of stochastic perturbations amplified in tissue regions that are near to bifurcation points. Therefore, a deterministic Luo-Rudy ventricular tissue (Faber & Rudy, 2000) was stochastically perturbed to determine the probability and conditions for spontaneous initiation of an ectopic beat and reentry (Clayton et al. 2003). EADs can be caused by low current density of the fast and slow delayed rectifying K⁺ currents (I_Kr, I_Ks), and incomplete inactivation of fast Na⁺ current. M cells, with the least I_Ks density, following a long diastolic interval are most prone to EADs. Therefore, the maximum conductance of I_Ks (g_Ks) of a M cell was reduced to 83% of the standard value, just below the critical point of producing an EAD. Addition of a small noise current at this critical g_Ks can trigger or reduce EADs in a single cell (Figure 1A). However, this bifurcation point is sensitive to [Na⁺]_i: high [Na⁺]_i will push the critical g_Ks to a lower level. In a homogenous 1-D tissue, the critical level of g_Ks is further reduced due to the electronic spread of membrane potential. The propagating wave develops ectopics as g_Ks is reduced to 74.8%. An ectopic can be a localised EAD or a bi-directional source, i.e. a focus. With g_Ks at 74.815%, a small noise current (± 0.1 mAcm^-2) evokes unidirectional propagations: (1) antegrade, equivalent to an ectopic; (2) retrograde, equivalent to a reentrant source. Fluctuations in membrane potential triggered ectopics can initiate reentry. However, such potentially reentrant activity occurred with a low probability (3 out of 71 cases), and was confined in a narrow range within the parameter space (0.03%) (Fig. 1B).