&NA;To assess whether coronary spasm affects the progression of atherosclerosis and results in evolution of myocardial infarction, the role of coronary spasm on the fine structure of conduit coronary arteries was studied morphologically. Göttingen miniature pigs were fed a semisynthetic diet containing 2% cholesterol and 1.1% sodium cholate. One month after being on this diet, the pigs were anesthetized and the endothelium of a branch of the left coronary artery was denuded using a balloon catheter. X‐ray irradiation in a dose of 1,500 rad was given twice selectively to the area denuded, after 4 and 5 months of cholesterol feeding. Five months after endothelial denudation, transient (group A) and repetitive episodes (group B) of coronary spasm were provoked by single and periodic (five times every 5 minutes) intracoronary injections of serotonin (10 &mgr;g/kg/injection), respectively. The extent of spasm by serotonin at the previously denuded site was 84±4% (n=4) and 90±5% (n=6) narrowing in groups A and B (p=NS between groups), respectively. Forty minutes after the final administration of serotonin, the left coronary artery was relaxed by nitroglycerin, and the heart was isolated and perfuse‐fixed under physiological pressure. Intramural hemorrhage was noted at the spastic site in six pigs of group B but not in group A. The average percent luminal narrowing, on cross sections at the spastic site in group B, was significantly greater than in group A (56±7% vs. 23±5%,p<0.01). Scanning electron micrographs revealed that the endothelial lining was intact at the nonspastic site in both groups. In addition to the appearance of intercellular bridges at the spastic site in both groups, squeezing of endothelial cells and adhesion of white blood cells were present at the spastic site exclusively in group B. These findings are consistent with the hypothesis that repetitive spasm may have an important role in the progression of atherosclerosis and/or myocardial infarction. (Circulation Research1989;65:272‐282)