Captopril was tested as the treatment for a patient with primary pulmonary hypertension (PPH) and its effects were compared with those of hydralazine. Captopril induced a rise in pulmonary pressures and in intrapulmonary shunt; hydralazine lowered pulmonary resistances and increased paO2 and blood O2 transport. Prospective studies in PPH treated with captopril are recommended and evaluation of all drugs not only by hemodynamic but also respiratory and O2 transport measurements.