Objective To assess the effect of sodium nitroprusside (SNP)-induced hypotension on gastric intramucosal pH (pHi), gastric intramucosal PCO2 ( PgCO2 ) and the difference between PgCO2 and PaCO2 (PgCO2-PaCO2) .Methods Fifteen ASA I patients (11 male, 4 female) aged 22-43 yr8, weighing 51-68 kg undergoing laminectomy for decompression of spinal cord in prone position were enrolled in this study. The patients were intubated and mechanically ventilated with 100% O2 after induction of general anesthesia. Anesthesia was maintained with isoflurane (1.2-1.4%) . Radial artery was cannulated for BP monitoring and boood sampling. TRIP tonometry catheter 14 F was inserted into stomach and connected to Tonocap ( Datex-Ohmeda, Finland) . Controlled hypotension was induced with SNP infusion at 1-5 ?g kg-1 min-1 . MAP was reduced to (60 ?5) mm Hg in 5-7 min and maintained for 60 min. Blood samples were taken before induced hypotension (T, ) , at 30 min (T2 ) and 60 min of hypotension (T3) and 30 min after hypotension (T4) for blood gas analysis. pHi, PgCO2 -PaCO2 and PETCO2 were also determined.Results MAP decreased by 35% and 33% at T2 and T3 as compared with the baseline (T, ) . pHi decreased significantly and PgCO2, PgCO2-PaCO2 increased significantly at T, as compared with those at T( ( P 0.05). pH at T2 ,T3 and T4 and BE at T3 and T4 were significantly lower than the baseline value (T1). PaCO2 and PET CO2 were significantly higher at T4 than those at T, (P