Ergonovine has become an accepted agent for the provocation of coronary artery spasm. Standard testing protocols generally use either cumulative or single intravenous bolus doses of ergonovine. A disadvantage of such protocols is that spasm may occur in both coronary arteries simultaneously, or during catheter exchange and, if severe, may be difficult to document and manage in some patients with severe myocardial ischemia, especially if using the Judkins technique. Recently, some investigators1,2 have begun to administer intracoronary ergonovine to allow selective provocation of spasm in a single coronary artery. It has been postulated that this method might offer more precise control of the vasoconstrictor stimulus and a wider margin of safety than is possible with intravenous administration of ergonovine. In addition, this approach may avoid systemic hypertension and occasional systemic adverse effects that may be seen with intravenous ergonovine. Despite increasing use of intracoronary ergonovine administration in patients, it has not been determined whether such administration causes a truly selective vasoconstrictor effect. The present study determines whether intracoronary ergonovine administration causes selective unilateral coronary artery constriction or whether the contralateral coronary artery is affected as well. © 1990.