Ventricular arrhythmias were induced by acute coronary artery ligation in anesthetized rats. 5-Hydroxytryptamine (5-HT) in doses of 100-200 micrograms kg-1, given i.v. 5 minutes before coronary artery ligation, enhanced the severity of ventricular arrhythmias as shown by a significant dose-dependent increase in the number of ventricular ectopic beats, duration of ventricular tachycardia (VT) and ventricular fibrillation (VF). This effect was antagonized by pretreatment with the selective 5-HT2-receptor antagonist ritanserin (1 mg kg-1, i.p.) given 15 minutes before 5-HT. Ritanserin when used alone was observed to produce a significant reduction in the number of ventricular ectopic beats and duration of VT and VF. 5-HT significantly lowered the heart rate and produced initial rise of the systolic blood pressure (SBP). These effects were antagonized by ritanserin. Ritanserin significantly lowered the heart rate but did not alter the SBP. It was postulated that 5-HT might be implicated in the genesis and determination of severity of ventricular arrhythmias induced by acute myocardial ischemia, and that this effect is mediated via 5-HT2-receptors. 5-HT2-receptor antagonists may provide potential useful therapeutic agents for the management of these arrhythmias.