Twenty post-open heart surgery patients with low output syndrome were given dopamine, sodium nitroprusside (SNP) alone and in combination. Dopamine alone (3--4 mcg/Kg/min) caused an increase of cardiac output (CO) from 3.2 to 4.6 L/min/M2 (p less than 0.001), SNP (1--1.5 mcg/Kg/min) raised the CO to 3.7 L/min/M2 (p less than 0.005). While the combination of the 2 drugs elevated the CO to 5 L/min/M2 (p less than 0.001). The mean pulmonary artery wedge pressure dropped moderately with dopamine and significantly with SNP and combined drugs. The diastolic pulmonary artery pressure fell significantly with either drug and in combination. The stroke index increased significantly with dopamine and combined therapy. All patients survived. It is concluded that in post-open heart surgery patients with low output syndrome substantial hemodynamic improvement results with the combined use of dopamine and SNP more than with either agent alone.