Digitized M-mode echocardiograms of left ventricular function were obtained in 34 patients subsequent to surgical repair of tetralogy of Fallot and in 34 healthy subjects at rest. In 16 patients and 16 controls, studies were also made during submaximal semisupine bicycle exercise. At rest, the peak ejection rate and fractional shortening were slightly reduced in the patients, whereas peak filling rate was comparable in patients and controls. Pulmonary regurgitation did not seem to influence left ventricular function. During exercise, however, peak filling rate was reduced in the patients compared to the healthy subjects. At peak exercise, the peak filling rates were 27.8±6.3 cm•s−1 and 34.1±3.4 cm•s−1 in patients and controls, respectively (p<0.01). No differences were found in heart rate, fractional shortening, peak ejection rate or blood pressures between patients and controls throughout the exercise test. When the subjects were subdivided by median age, the oldest patients had reduced normalized peak filling rates throughout the exercise test, whereas no differences were found between younger and older healthy subjects. Thus, it is suggested that the reduced left ventricular peak filling rate found during exercise is caused by subclinical myocardial dysfunction which seems to be related to myocardial protection at surgery and the period of follow-up.