An interactive computer program, LabHEART, was developed to simulate the action potential (AP), ionic currents, and Ca handling mechanisms in a rabbit ventricular myocyte. User-oriented, its design allows switching between voltage and current clamp and easy on-line manipulation of key parameters to change the original formulation. The model reproduces normal rabbit ventricular myocyte currents, Ca transients, and APs. We also changed parameters to simulate data from heart failure (HF) myocytes, including reduced transient outward (I _to) and inward rectifying K currents (I _K1), enhanced Na/Ca exchange expression, and reduced sarcoplasmic reticulum Ca-ATPase function, but unaltered Ca current density. These changes caused reduced Ca transient amplitude and increased AP duration (especially at lower frequency) as observed experimentally. The model shows that the increased Na/Ca exchange current (I _NaCa) in HF lowers the intracellular [Ca] threshold for a triggered AP from 800 to 540 nM. Similarly, the decrease in I _K1reduces the threshold to 600 nM. Changes in I _tohave no effect. Combining enhanced Na/Ca exchange with reducedI _K1 (as in HF) lowers the threshold to trigger an AP to 380 nM. These changes reproduce experimental results in HF, where the contributions of different factors are not readily distinguishable. We conclude that the triggered APs that contribute to nonreentrant ventricular tachycardia in HF are due approximately equally (and nearly additively) to alterations inI _NaCa and I _K1. A free copy of this software can be obtained at http://www.meddean.luc.edu/lumen/DeptWebs/physio/bers.html .