To assess the effect of prostacyclin on the diseased coronary circulation basally and, in particular, on the coronary responses to the cold pressor test, a small dose of 4 ng/kg/min and a large dose of 8 to 10 ng/kg/min was infused in 11 patients with stable angina pectoris. Coronary blood flow was measured by coronary sinus thermodilution technique. The mean blood pressure decreased from 97 +/- 5 to 89 +/- 5 mm Hg during the low-dose infusion (p less than 0.005) and to 81 +/- 5 mm Hg during the high-dose infusion (p less than 0.001); the heart rate increased from 65 +/- 4 to 69 +/- 4 beats/min during the low-dose infusion (p less than 0.05) and to 78 +/- 5 beats/min during the high-dose infusion (p less than 0.001). Systemic vascular resistance decreased by 11 +/- 4% with small doses (p less than 0.05) and by 38 +/- 4% with large doses (p less than 0.001) of prostacyclin, and coronary vascular resistance decreased by 16 +/- 7% (p less than 0.05) with the small dose and by 29 +/- 6% (p less than 0.001) with the large dose of prostacyclin. Seven of 11 patients showed a baseline vasoconstrictor response to the cold pressor test (increase in coronary vascular resistance of 11 +/- 2%). This increase in coronary vascular resistance was not altered by either the small or the large dose of prostacyclin. Thus, prostacyclin causes marked coronary and systemic vasodilation, with no evidence of selective enhancement of the sensitivity of the diseased coronary circulation.(ABSTRACT TRUNCATED AT 250 WORDS)