In order to determine whether exogenous coenzyme Q can modify the course of mechanical changes in the myocardium during hypoxia and reoxygenation, we studied isolated rat heart muscles contracting isometrically at Lmax, 12 times per minutes at 28 degrees C. The prehypoxic mechanical parameters of 6 control muscles and 6 muscles treated with coenzyme Q10 (40 m micrograms/ml in the bath) did not differ, nor did the changes in developed tension and resting tension during 15 min of hypoxia and 45 min of reoxygenation. However, the time it took for the tension to fall to 50% of the peak value (RT1/2), the time from the onset of contraction to peak isometric tension (TPT) and TPT + RT1/2 on early reoxygenation were significantly diminished in coenzyme Q treated muscles. Thus, the tension prolongation phenomenon was attenuated in preparations of coenzyme Q treated rat heart muscle. It is concluded from this study that exogenous coenzyme Q does not modify the mechanical performance of isolated rat heart muscle during hypoxia but significantly attenuates the tension prolongation phenomenon. Since oxygen free radicals are known to induce mitochondrial dysfunction and impairment of calcium sequestration in the sarcoplasmic reticulum, the mechanism by which coenzyme Q attenuates the tension prolongation phenomenon is probably attributable to its function as an antioxidant.