BACKGROUND: Vasoconstricting drugs such as dopamine, phenylephrine (PE) and epinephrine constrict normoxic lung vessels preferentially, thereby disproportionately increasing normoxic lung pulmonary vascular resistance (PVR) and inhibit hypoxic pulmonary vasoconstriction (HPV). In this study, we evaluated the effect of PE on HPV and arterial oxygenation. METHODS: This study was performed on 21 patients undergoing thoracotomy. After induction of anesthesia, Swan-Ganz catheter was inserted. After one lung ventilation was started, systolic blood pressure (SBP) of the patient was reduced to 100 mmHg using inhalation anesthetic agent and then the blood pressure was raised up to 140 mmHg by PE infusion. Hemodynamic variables were measured and arterial blood gas was analyzed at the start of one lung ventilation (control), SBP of 100 mmHg and SBP of 140 mmHg. RESULTS: The mean dose of PE infused was 5.9 +/- 3.8 microgram/kg. Infusion of PE did not increase pulmonary vascular resistant index (PVRI) significantly and did not reduce arterial PO2. There was no statistically significant difference in intrapulmonary shunt fraction (Qs/Qt) between the time of low and high blood pressures. CONCLUSION: Pulmonary vasomotor changes induced by PE are minimal and so should not affect the distribution of blood flow during one lung ventilation. On the basis of this result, PE appears to a reasonable vasoconstrictor to be used in patients undergoing thoracotomy.