This study was designed to characterize the changes in left ventricular performance induced by graded isometric exercise. Fourteen healthy subjects (12 men and 2 women), aged 19 to 27, performed handgrip isometric exercise at 20, 40, and 60 percent of their maximal voluntary contraction (MVC) with three minutes of rest interval between each contraction. Left ventricular performance was assessed by M-mode echocardiography at rest and during each isometric contraction. Left ventricular end-diastolic and end-systolic dimensions did not change significantly. Heart rate and blood pressure increased significantly during each level of isometric contraction. Left ventricular posterior wall thickness fell from 8.6 ± .4 (mean ± SE) to 7.1 ± .5 (P<0.01) and 6.5 ± .4 mm (P<0.001) in response to 40 and 60 percent of MVC, respectively. Left ventricular wall thickness to radius ratio decreased progressively as systolic blood pressure increased which suggests increased wall tension. Fractional shortening remained unchanged during graded isometric exercise. However, mean velocity of circumferential fiber shortening (mVcf) increased from 1.08 ± 0.4 to 1.24 ± .05 circ/sec (P<0.005) in response to 60 percent of MVC. The authors conclude that the effect of isometric exercise in left ventricular performance depends, to some extent, on the relative strength of muscle contraction. In healthy subjects, isometric exercise of low intensity (<40 percent of MVC) does not generally result in depression of left ventricular function. Higher levels of isometric exercise may modestly enhance left ventricular performance despite a sudden increase in blood pressure.