BACKGROUND: A decrease in mean arterial pressure (MAP) often accompanies aortic declamping during open heart surgery (OHS). But, there are no many reports to evaluate the extent of the decrease in MAP following the aortic declamping during OHS. METHOD: Twenty nine patients undergoing OHS, 17 patients for repair of cushion defect and 12 patients for valve replacement were evaluated. Anesthesia was provided by fentanyl (30 g/kg bolus followd by 0.3 ug/kg/min infusion) and intermittent inhalation of N2O-Isoflurane. An indwelling radial artery catheter was used to measure MAP. Systemic vascular resistance (SVR) was calculated using the formula. Measurements were made just before aortic declamping (control), and 1,3,5,7 and 10 minutes after declamping. RESULTS: MAP and SVR were decreased significantly until 7 minutes after aortic declamping compared to those of control value, and have a significant difference in decrease pattern of MAP following time interval after declamping between the patients for repair of cushion defect and the patient for valve replacement. The extent of decrease in MAP and/or SVR showed no significant relationship with difference the duration of bypass, patients body temperature and age. CONCLUSION: The results of this study indicated that there is a significant decrease in MAP and SVR after aortic declamping and it persists for about 7 minutes, without significant relation with bypass time, body temperaure and age.