10 healthy men aged 18 to 32 years underwent M-mode-echocardiographic studies at rest, during exercise and recovery. Semisupine bicycle exercise was performed using work loads of 25, 50, 75, and 100 W with measurements taken every minute. With increasing exercise, heart rate rose significantly (p less than 0.01) from 68/min to a maximum of 132/min, blood pressure as obtained by sphygmomanometry rose from a mean of 83 torr to 102 torr. Left ventricular enddiastolic dimension (EDD) did not change significantly with exercise, left ventricular endsystolic dimension (ESD) decreased from a group mean of 34.8 mm to 30.5 mm. This paralleled an increase in stroke dimension from 18.3 to 22.6 mm and of fractional shortening (FS) from 0.34 to 0.43. Mean velocity of circumferential fiber shortening (MVCF) rose from 1.01 circ/s to 2.06 circ/s. During recovery, heart rate and blood pressure returned faster to resting levels than did ESD, FS, and MVCF. Again EDD did not change. These data indicate that heart rate contributes more to the increases in cardiac output observed with ergometric exercise than does stroke volume; with moderate exercise levels this increase in stroke volume is produced by diminishing endsystolic volume whilst enddiastolic volume remains unchanged.