Left ventricular performance was studied in 9 well-trained athletes at rest, during maximal exercise, and during the recovery period following exercise. All athletes showed an expected response to maximal bicycle exercise in the supine position. During the recovery period signs of impaired diastolic as well as systolic performance were noted. Abnormal filling characteristics and relatively increased duration of systole at the expense of diastole prevailed 25 min after exercise was stopped, reflecting a decrease in left ventricular diastolic distensibility and a compromised contractility. These findings were paralleled by signs of myocardial oxygen debt/increased oxygen consumption during the recovery period. At the last recovery recording (25 min after exercise), the athletes had a ventricular function which showed some resemblance to the characteristics of left ventricles with concentric hypertrophy. The study suggests that the athletes succumbed to myocardial oxygen debt up to 25 min after maximal exercise.