Systolic time intervals (STI) were analyzed in 34 patients with isolated ventricular septal defect (VSD) and undirectional left-to-right shunt. 14 of the patients who underwent corrective surgery were followed-up for at least 10 years. The measurements were obtained from simultaneous high speed photographic recordings of electrocardiogram, external carotid pulse and phonocardiogram. Before the operation the Q-I sound interval and, consequently, the pre-ejection period were significantly prolonged and the left ventricular ejection time significantly abbreviated, the degree of abbreviation relating with the magnitude of the shunt. The above deviations persisted after corrective surgery, and only several years after the operation the parameters studied became normal. It is concluded that the most likely explanation for these abnormalities is the depressed contractility of the left ventricle, secondary to the long-standing volume overload, which tends to persist after corrective surgery.