Consistent threshold concentrations were determined for the precipitation of spontaneous ventricular fibrillation in isolated rabbit hearts by reduction of potassium or increase of calcium in the perfusion solution. The low potassium threshold was a function of the calcium concentration: it increased from 0.81 +/- 0.14 mM (mean +/- SD) with 1.0 mM calcium to 1.58 +/- 0.17 mM with 5.0 mM calcium. The high calcium threshold was 12.8 +/- 2.1 mM at 5.6 mM potassium. Consistent thresholds could not be established when the calcium concentration was greater than 5 mM and potassium concentration less than 3.6 mM. The absolute low potassium and high calcium thresholds did not depend on the potassium and calcium concentrations of the control perfusion solution, being the same whether these were "conventional" (5.6 and 2.25 mM, respectively) or "physiological" (3.6 and 1.6 mM). Spontaneous fibrillation appeared sooner when precipitated by high calcium than when precipitated by low potassium. Many other arrhythmias were observed; these were neither sufficient nor even necessary precursors of spontaneous fibrillation. Particularly with high calcium, spontaneous fibrillation could be the first arrhythmia to appear. Hearts perfused with hypertonic McEwen's solution were more susceptible to the precipitation of fibrillation by low potassium than those perfused with more physiologically balanced solutions. Enhanced susceptibility to precipitation of fibrillation by high calcium concentration was not seen.