BACKGROUND: It has been reported that two common vasodilators, nitroglycerin (NTG) and sodium nitroprusside (SNP), inhibit regional HPV and decrease arterial oxygenation as a result. The purpose of the present study was, therefore, to determine the comparative effect of NTG and SNP on HPV in a rabbit model of isolated lung perfusion with exclusion of the influential factors on HPV. METHODS: In adult white rabbits (n=20), lungs were isolated and perfused with the constant pulmonary blood flow. The acid-base status and temperature of perfusate was also constantly maintained. Thirty minutes later, baseline hypoxic pressor response (HPR) was measured as the difference of pulmonary artery pressure (PAP) between a period of 95% hyperoxic gas inhalation and that of 3% hypoxic gas inhalation. ED50 of NTG and SNP was calculated from the hypoxic pressor response measured in the same way, according to changes of doses (0.5, 1.0, 2.0, and 5.0 microgram/kg). RESULTS: Both NTG and SNP significantly decreased the baseline PAP in the doses of 1.0 microgram/kg and above, and also decreased the HPR in a dose-related manner. ED50 of SNP was significantly lower than that of NTG. CONCLUSIONS: NTG and SNP dilated directly the pulmonary vasculature and inhibited HPV in a dose- related manner. SNP had a greater inhibiting effect on HPV than NTG.