Two groups of eight dogs each received injections of olive oil (1.5 ml/kg) after control measurements were taken. One group was pretreated with reserpine (0.25 mg/kg) for two days. Both groups had the same initial changes in hemodynamics and pulmonary function. However, by three to five hours the reserpine-treated group clearly had less arterial hypoxia (oxygen pressure [PAO2] 77.1 vs 55 mm Hg) on the basis of a smaller calculated pulmonary shunt (9.92% vs 14.9%), and on the basis of a higher cardiac output (63% vs 55% of initial value).