The effects of acetylcholine, 10 microgram/kg i.v., were examined in nine conscious dogs on measurements of left ventricular pressure, dP/dt, aortic pressure, heart rate, coronary blood flow, left circumflex external coronary arterial diameter, arterial and coronary sinus O2 content and calculations of late diastolic coronary resistance and left circumflex internal coronary cross-sectional area. In conscious dogs in the absence of autonomic blockade, acetylcholine induced a rapid, transient response characterized by hypotension and peak increases in coronary blood flow (+135 +/- 14%) and coronary sinus O2 content (+5.0 +/- 0.5 volume percent) and decreases in late diastolic coronary resistance (-65 +/- 3%). The peak large coronary arterial effects were observed 60 sec later at a time when arterial pressure, left ventricular end-diastolic pressure, left ventricular dP/dt and heart rate were not significantly different from control. At this time, large coronary cross-sectional area was increased by 36 +/- 4%, whereas late diastolic coronary resistance was not significantly different from control. Beta adrenergic or combined alpha and beta adrenergic receptor blockade did not alter the responses significantly. However, the effects were abolished by muscarinic blockade with atropine methylbromide, 0.1 mg/kg. Thus, acetylcholine dilates large coronary arteries as well as coronary resistance vessels. These effects are independent of metabolic and adrenergic mechanisms, and are mediated by muscarinic receptors, inasmuch as the effects are abolished by prior muscarinic receptor blockade.