Thirteen patients who had mitral valve surgery were studied within 3 hours after operation. The patients were divided into 2 groups: group A with initial low cardiac index (1.70 ± 0.25), elevated left atrial pressure (16.5 ± 6.7) and high peripheral vascular resistance (2623 ± 789); group B with initial normal cardiac index (3.71 ± 0.54), normal left atrial pressure (13 ± 3.5) and normal peripheral vascular resistance (1223 ± 303). In both groups the mean arterial pressure was elevated (98 ± 8.8, 96 ± 15.8). An infusion of nitroprusside to reduce the mean arterial pressure to either 80 mmHg or 10% below the initial value had different effects in each group. In group A, cardiac index (CI) increased by 23%, left atrial pressure (LAP) decreased by 20%, pulmonary artery pressure (PAP) by 35%, and peripheral vascular resistance (PRV) by 32%. In group B, CI decreased by 8%, LAP by 32%, PAP by 30% and PRV by 13%. When LAP returned to initial values after an infusion of blood with continued infusion of nitroprusside, CI increased in both groups (27%, 11%) and PRV remained lower (40%, 29%). The study demonstrates the favorable effect on cardiac output of vasodilator therapy on patients with elevated blood pressure, impaired left ventricular function and high LAP, after surgery on mitral valve. The optimal effect is achieved by keeping the LAP within normal limits.