Chronic effects of coenzyme Q10 (CoQ: 30 mg/kg/day for 12 weeks) on cardiac performance in streptozotocin (45 mg/kg) induced diabetic rats were examined. Cardiac performance was assessed using the isolated retrograde perfused isovolumically contracting heart model. Compared to age matched nondiabetic rats, decreases occurred in myocardial CoQ (25.8 +/- 3.3 vs. 31.9 +/- 2.7 micrograms/ml), AMP (0.9 +/- 0.7 vs. 2.0 +/- 0.4 micrograms/mg), Emax (37 +/- 14 vs. 80 +/- 38 mmHg/microliter/g), an index of myocardial contractility, and LV diastolic chamber stiffness constant k (0.68 +/- 0.13 vs. 1.31 +/- 0.59 g/microliter) in diabetic rats. Normalized left ventricular weight (2.97 +/- 0.23 vs. 2.51 +/- 0.21 mg/g) and volume (1.53 +/- 0.34 vs. 0.89 +/- 0.53 microliter/g) and time constant of left ventricular pressure fall, T (32.0 +/- 8.0 vs. 19.7 +/- 2.6 ms) increased in diabetes. In diabetic rats taking CoQ, myocardial CoQ (28.5 +/- 3.2 micrograms/ml) and AMP (2.1 +/- 1.7 micrograms/mg) were the same as control, and T (23.5 +/- 7.4 ms) was significantly shortened (mean +/- SD, p less than 0.05, p less than 0.01). To compensate for depressed myocardial contractility and relaxation, LV dilatation and increased LV mass occurred in diabetic rats. Exogenous CoQ increased myocardial CoQ content and improved myocardial relaxation in diabetic rats.