Instruments were inserted to seven dogs under halothane anaesthesia, to measure global and regional left ventricular function. Anaesthesia was continued with fentanyl (100 micrograms kg-1 bolus, then 1.5 micrograms kg-1 min-1). Critical constriction was applied to the left anterior descending coronary artery. Control recordings were made, followed by bolus administration of verapamil 0.08, 0.16 and 0.32 mg kg-1, with recordings 10 min after each bolus. At the highest dose, verapamil decreased systemic arterial pressure, left ventricular dP/dt, stroke volume and systemic vascular resistance, and increased heart rate significantly. Coronary perfusion pressure decreased and, in the presence of critical constriction, coronary flow per beat decreased significantly. In the region with constriction, systolic shortening of myocardium decreased and post-systolic shortening increased significantly with addition of verapamil. The addition of a high dose of verapamil to fentanyl anaesthesia caused reduction in systolic function and development of early diastolic dysfunction in myocardium with critically limited blood supply.