In open chest dogs under sodium pentobarbital anaesthesia the interaction of mechanical constriction on a large coronary branch and autoregulatory capacity of the relevant small resistance vessels was analyzed. Coronary blood flow (CBF) was measured with an electromagnetic flowmeter. Step-by-step mechanical constriction gradually abolished adenosine-induced coronary vasodilation, whereas the resting level of mean CBF remained unaltered. At this point verapamil (0.2 mg/kg i.v.), a vasodilator with a strong potency of blocking adenosine action, eventually decreased CBF and increased coronary resistance. Similar results were obtained with these drugs injected directly into a bypass established between the carotid and left common coronary arteries. The results suggest that (i) adenosine affects the same coronary segments which accomplish compensatory autoregulation (ii); with critical stenosis verapamil augments indirectly coronary resistance by inhibiting an "intrinsic" adenosine effect (iii); the functional state of stenosed coronaries can be assessed with the aid of these pharmacologic tests.