 If ET-1 is important in the development of unstable angina, then medications which inhibit the effects of ET-1 should improve the condition. To achieve a blockade of ET-1, inhibitors of the two receptors ET-A and ET-B responsible for ET-1s action must be administered. Animal and human studies have demonstrated that a blockade of the ET-A receptor by the new antagonist, BQ-123, inhibits most of the ET-1 induced constrictor response. BQ-123 has been safely administered systemically by intravenous infusion and locally in the human forearm where it produces dilation of the forearm arteries. It has not been previously administered into human coronary arteries. 