The effects of ischaemia on monophasic action potential duration and conformation were studied using suction electrodes on the in situ left ventricle in anaesthetised pigs. Action potentials were recorded throughout the first hour of ischaemia and their behaviour studied on a beat to beat basis by computer analysis. The natural history of action potential duration changes in the ischaemic area was based on the presence or absence of four events. Typically, ischaemic segments showed a brief increase in action potential duration after coronary ligation (event 1), followed by a rapid fall in action potential duration (event 2), a temporary, partial, spontaneous recovery of action potential duration after about 15 min (event 3), and, finally, a decay towards inexcitability of the segment (event 4). An increase in variability of action potential duration was associated with event 2, sometimes manifest as electrical alternans. Simultaneous records from control segments of non-ischaemic myocardium were relatively unchanged. Segments of myocardium bridging the cyanotic border sometimes showed the events. Non-uniform or dispersed event 3 in ischaemia, coupled with the increased beat to beat variability after event 2, could establish the conditions for ventricular arrhythmias and fibrillation.