The present study examined the effect of prostacyclin (PGI2) on ventricular arrhythmias following coronary artery occlusion in the cat. The left anterior descending coronary artery (LAD) was occluded abruptly in 50 cats anesthetized with alpha-chloralose. The ECG (Lead II) along with arterial blood pressure were monitored before and for at least one hour after occlusion. Either vehicle or PGI2 was infused at a rate of 0.15 ml . min-1 into the left atrium 15 min before and 1 hour after LAD occlusion. PGI2 was infused at 4 doses: 2.7,27,270 and 2700 pmole . kg-1 . min-1. Infusion of PGI2 before occlusion produced a dose dependent decrease in mean arterial blood pressure with no significant change in heart rate. Abrupt occlusion of the LAD produced ventricular arrhythmia in all cats ranging from ventricular premature beats to ventricular fibrillation (VF). In cats infused with PGI2 the incidence of VF ranged from 30-40% at the low and middle doses to a maximum of 60% at the highest dose (2700 pmole . kg-1 . min-10. The incidence of VF in the latter group was two times greater than that observed in the control group (30%). In addition, the mean number of ventricular ectopic beats was greater at the 27 and 2700 pmole . kg-1 . min-1 doses of PGI2 than in the control group. An increase in ventricular ectopic beats was not observed at the 2.7 or 270 pmole . kg-1 . min-1 doses of PGI2. These data indicate the PGI2 can exert an arrhythmogenic effect following coronary artery occlusion and that this effect occurs in a biphasic manner i.e., the increase in arrhythmia observed at 27 did not occur at 270 but occurred again at the 10 fold higher dose (2700 pmole . kg-1 . min-1). At the lowest dose of PGI2 infarct size was approximately 14% less than in the control group while, at the highest dose, infarct size was 20% greater than control and approximatey 40% greater than the lowest dose of PGI2.