Twenty-one patients with various heart diseases undergoing the coronary sinus catheterization had myocardial blood flow studies before and during isometric handgrip exercise. At 30% of the maximal voluntary contraction (MVC), handgrip increased both coronary sinus blood flow by 19.8% and myocardial O2 consumption by 21.0% on the average as compared to those at rest. At 20% of MVC, the increase of the both was slight except for 1 case. The increase in coronary sinus blood flow significantly correlated to the increase of myocardial O2 consumption. Lactate extraction ratio decreased to less than 10% during handgrip in 4 of 19 cases studied despite of normal values at rest. Lactate extraction ratio had significantly positive correlations with coronary sinus blood flow and myocardial O2 consumption during handgrip, while there were not significant ones at rest. It is suggested that isometric handgrip exercise is useful as a stress test for detection of myocardial ischemia in the heart of coronary artery disease.