OBJECTIVE Dispersion in refractoriness is considered a major factor in induction and persistence of cardiac arrhythmias. The sympathetic nervous system is known to modulate refractoriness. An index of refractoriness has therefore been assessed in normal and ischaemic myocardium simultaneously at multiple sites, with and without sympathetic stimulation. METHODS In six dogs on total cardiopulmonary bypass the average interval between local activations was measured during artificially induced ventricular fibrillation from extracellular electrograms simultaneously recorded from 32 ventricular sites. These local ventricular fibrillation intervals may be used as an index of local refractoriness. RESULTS During regional ischaemia, ventricular fibrillation intervals of ischaemic sites could prolong by up to 60% after 3 min following coronary occlusion. Left stellate ganglion stimulation during ischaemia produced either no response or prolonged the ventricular fibrillation intervals even further at ischaemic sites, whereas ventricular fibrillation intervals at non-ischaemic sites shortened. Dispersion in refractoriness across the ischaemic border increased by 14-59% in individual hearts following sympathetic stimulation during acute, regional ischaemia. CONCLUSIONS Due to opposite effects on normal and ischaemic myocardium, sympathetic stimulation increases the difference in refractoriness over the ischaemic border. This may enhance the chance for regional conduction block and the propensity to re-entrant arrhythmias.