Reproducible left ventricular dimensions were found by M-mode echocardiography in eight healthy men in the semisupine position during two maximal bicycle exercise tests, performed with four hours interval. Left ventricular end-diastolic dimension did not increase during maximal exercise, while fractional shortening increased by a decrease in end-systolic dimension. Twelve men studied by the same procedure were given 100 mg atenolol orally just after the first test which conspicuously reduced their heart rate response to exercise. End-diastolic dimension increased significantly from rest to peak exercise after the administration of atenolol in contrast to that before beta blockade, and fractional shortening at maximal exercise increased compared with the preceding control test. We conclude that atenolol changes the left ventricular response to maximal semisupine exercise in normal man, with dilatation and a concomitant increase in systolic myocardial shortening. This suggests that atenolol during maximal exercise reveals the part played by the Frank-Starling mechanism in cardiac reserve. In addition to that mechanism, the increased ventricular emptying is probably also the result of reduced afterload after administration of atenolol.