Under continuous ECG and oxygen saturation (SpO ₂) monitoring, the following measurements were taken by Doppler echocardiography in 6 consecutive patients with tetralogy of Fallot (TF) before and after intravenous administration of disopyramide (2 mg/kg): left ventricular shortening fraction (LVSF); peak velocities in the right ventricular outflow tract (RVOT); diastolic and systolic internal diameters of the right ventricular outflow tract (dRVOT, sRVOT); and systolic blood pressure. SpO₂ increased (p<0.01) from 78 to 98 (89±7, mean ± standard deviation)% to 86-99 (94±5)%. LVSF decreased (p<0.05) from 0.34-0.56 (0.42±0.08) to 0.22-0.54 (0.33±0.13). The systolic blood pressure fell slightly (p<0.05) from 68-92 (79±8) to 64-92 (71±11) mmHg. The sRVOT increased (p<0.05) from 2.1-4.8 (2.7±1.5) mm to 3.0-8.1 (4.9±2.4) mm, while RVOT peak velocity decreased (p<0.05) from 2.20-4.88 (3.70±0.97) m/sec to 2.05-4.07 (2.92±0.72) m/sec. Disopyramide alleviates hypoxia in patients of TF through its negative inotropic action on right ventricular outflow obstruction. (Jpn Circ J 1998; 62: 807 - 810)