Abstract We studied the effect of intravenous prostacyclin (PGI 2) infusion on epinephrine exacerbation of platelet-mediated acute coronary thrombus formation in an in vivo canine model of coronary artery stenosis. Platelet thrombi form in mechanically stenosed dog circumflex coronary arteries, producing cyclical reductions in coronary blood flow (CRF) as measured with an electromagnetic flowmeter probe. In the present study nine of 10 dogs exhibited spontaneously occurring CRF. With epinephrine (E) infusions of 10 μg/min CRF frequency increased 64% (p < 0.025), CRF magnitude increased 27.1% (p < 0.05), and the rate of flow decline increased 112.8%, indicating that the rate of thrombus formation increased with E infusion (p < 0.005). When an identical E infusion was accompanied by simultaneous PGI 2 infusion (150 ng/kg/min), CRF were abolished in nine of 10 dogs and markedly inhibited in the other. After the 10 min of simultaneous E and PGI 2 infusion, E infusion alone was continued. There was a recurrence of CRF within 2.49 ± 0.89 min after cessation of PGI 2 infusion. However, CRF frequency in the subsequent 10 min was less than the frequency of CRF during the initial E infusion (p < 0.10) and not significantly different from that of the control period (no E infusion). The rate of flow decline and magnitude during the final E infusion after cessation of PGI 2 infusion were not significantly different from those of the initial E infusion. Prostacyclin infusion in the coronary care unit may be potentially beneficial in cases of acute myocardial ischemia where elevated catecholamines are a thrombogenic stimulus.