Although the incidence of ventricular arrhythmias following myocardial ischaemia lessens as ischaemia improves, it is not clear whether this is correlated with a reduction in the degree of ST segment elevation. To explore this further we examined the effects of change in heart rate and the administration of the calcium antagonist diltiazem, 0.02 mg.kg-1.min-1, on ST segment elevation and the alternans of ST segment elevation (STA) and on serious ventricular arrhythmia induced by 10 min occlusion of the left anterior descending coronary artery in 86 mongrel dogs. The dogs were divided into three groups: 26 dogs paced at a rate of 180 beats.min-1 (group A); 44 dogs paced at a rate of 120 beats.min-1 (group B); and 16 dogs paced at a rate of 180 beats.min-1 and given diltiazem intravenously from 25 min before the coronary occlusion (group C). The degree of ST segment elevation and STA within 3 min of ischaemia was significantly lower in group B than in group A. There was no marked difference in the degree of ST segment elevation between groups A and C, but the STA was lower in group C than in group A. Incidence of ventricular tachycardia and ventricular fibrillation was significantly lower in groups B and C than in group A, and the timing of their first appearance was 4.5 (SEM 0.6), 4.2(0.9) and 3.0(0.4) min, respectively. We suggest that the reduction in serious ventricular arrhythmias associated with the decrease in heart rate was caused by the improvement of STA secondary to the improvement of ST segment elevation.(ABSTRACT TRUNCATED AT 250 WORDS)