Protection of the ischaemic myocardium with Coenzyme Q10 (COQ10) following constriction of left anterior descending coronary artery was studied in 38 open-chest mongrel dogs. Left anterior descending coronary artery blood flow was independently reduced by tightening a constrictor around the vessel. Aortic pressure and left ventricular pressure were measured, and the Tension Time Index (TTI) was calculated. Myocardial segment shortening in the ischaemic area was measured with an ultrasonic dimension gauge. After obtaining haemodynamic data, the ischaemic myocardium was biopsied and the tissue sample was frozen in liquid nitrogen for adenosine triphosphate (ATP) analysis. Animals premedicated with COQ10 (20 mg·kg−1 iv) had a significantly higher ATP content in ischaemic myocardium (3.25 ± 0.35 μmol·g−1 wet weight) compared with that in ischaemic myocardium of control animals (2.96 ± 0.19) (P < 0.05), when coronary blood flow was reduced by 50 to 74%. There was failure of ATP preservation by COQ10 with a further reduction in coronary blood flow. Changes in the pattern of myocardial segment shortening in the ischaemic area induced by a 50 to 74% coronary blood flow reduction were less in COQ10 treated animals than in control animals. Since there were no haemodynamic differences between control and COQ10 treated animals, these effects were considered not to be due to a decrease in myocardial oxygen consumption secondary to haemodynamic changes.