Effects of the experimentally induced hypotension with 4 vasodilators; nitroglycerin (TNG), prostaglandin E1 (PGE1), trimetaphan (TMP) and nicardipine (NCP) on systemic vascular resistance (SVR), pulmonary vascular resistance (PVR) and pulmonary-systemic vascular resistance ratio (PVR/SVR ratio) were studied in dogs. The SVR values were significantly reduced by TNG, PGE1 and NCP, and not affected by TMP. The PVR values were significantly reduced by TNG and PGE1, and not affected by TMP and NCP. The PVR/SVR ratio values were significantly reduced by TNG, and significantly increased by NCP, and not affected by PGE1 and TMP. We concluded that TNG reduced PVR and SVR but it affected PVR more; PGE1 reduced PVR and SVR equivalently; TMP did not affect PVR and SVR remarkably; NCP reduced SVR but did not affect PVR.