In order to evaluate the effects of intracoronary acetylcholine on coronary resistance vessels, oxygen saturation in coronary sinus blood was continuously measured to compare its dynamic changes during intracoronary injection of acetylcholine in both patients with variant angina and control subjects. Group 1 consisted of 6 patients without coronary artery disease. Group 2 consisted of 10 patients with variant angina and spasm in the left anterior descending coronary artery. A fiberoptic reflection oximetry system was used for the continuous measurement of coronary sinus venous oxygen saturation. Acetylcholine (20 µg) was injected directly into the left coronary artery over 30 s. In the group 1 patients, coronary sinus venous oxygen saturation was increased from 39 ± 2% (mean ± SEM) to 54 ± 3% at 30 s, continuously climbed to 70 ± 3% at 60 s and then gradually decreased to 53 ± 5% at 120 s after the initiation of intracoronary injection of acetylcholine. In contrast, in the group 2 patients, coronary sinus venous oxygen saturation was transiently increased from 39 ± 2% to 56 ± 4% at 30 s, reversed, decreased to 52 ± 4% at 60 s and then rapidly decreased to 36 ± 3% at 120 s with the onset of chest pain associated with electrocardiographic ischemic changes. Coronary arteriography during attacks demonstrated a total or subtotal occlusion of the left anterior descending coronary artery due to severe spasm in all of the 10 patients. The extent of increases in coronary sinus venous oxygen saturation at 30 s after acetylcholine injection was not significantly different between the two groups (group 1: 15 ± 4%, group 2: 17 ± 3%). Heart rate, blood pressure and rate-pressure product were essentially unchanged at 30 s after intracoronary injection of acetylcholine in both groups. These data suggest that in control adult humans, coronary blood flow was increased through dilatation of resistance vessels by acetylcholine, while in patients with variant angina, coronary blood flow was transiently increased by dilatation of resistance vessels, after which it was suddenly decreased by spasm of an epicardial artery induced by this agent. Relaxant responses to acetylcholine of coronary resistance vessels appear to be preserved well in patients with variant angina.