BACKGROUND: It is difficult to choose the appropriate drug when hypotension develops in patients with pulmonary hypertension (PH). There is no known drug to increase the systemic blood pressure (BP) without an increase of pulmonary arterial pressure (PAP). We observed the effects of phenylephrine (PE) and norepinehrine (NE) on systemic and pulmonary hemodynamics when hypotension was treated in patients with PH. METHODS: Patients with PH (mean PAP > or = 25 mmHg, n = 28) were studied. When hypotension occurred (systolic BP < or = 100 mmHg, T1) after the induction of anesthesia, PE or NE was randomly infused to raise the systolic BP above 130 mmHg (T2) and 150 mmHg (T3). Hemodynamic variables were measured at T1, T2 and T3, and the ratio (RBP) of mean PAP to mean BP was calculated. The measurements were performed before skin incision to avoid the effects of surgical stimulation. RESULTS: NE increased BP concomitantly with relatively small increase of PAP, meaning a decrease of RBP (P < 0.05) without any other changes of hemodynamic variables in all patients. However, PE could not raise BP above 130 mmHg in one third of patients, and decreased the cardiac index without a significant decrease of RBP. CONCLUSIONS: NE increased BP and decreased RBP without tachycardia or any other hemodynamic disturbances. NE is considered to be a proper and safe drug to raise BP when hypotension occurs in patients with PH.