Pulmonary vascular reactivity tests were performed in a young woman with mixed connective tissue disease and severe pulmonary hypertension. Vasoreactivity was documented in response to intravenous prostacyclin (PGI2), oral nifedipine, and inhaled nitric oxide, with quantitative differences. Nitric oxide produced a moderate lowering of pulmonary arterial pressure and resistance without any deleterious systemic effect. The use of nitric oxide in testing for pulmonary vasoreactivity merits further evaluation.