We have studied the effect of thiopental sodium, propofol, midazolam and ketamine on systemic vascular resistance(SVR) during cardiopulmonary bypass with constant pump flow in 20 patients undergoing elective open heart surgery. SVR decreased about 13(+/-3.42)% of control values after thiopental sodium 4 mg/kg, about 10 (+/-5.30)% of control after propofol 2 mg/kg and about 8(+/-3.72)% of control after midazolam 0.2 mg/kg; it returned to control values about 2 min 30 sec(+/-1 min 20 sec) after administration of thiopental sodium and about 4 min 30 sec(+/-2 min 15 sec) after administration of propofol. It remained under control values after 10 min after administration of midazolam. Ketamine showed no significant changes on SVR. Analysis of variance showed that there were no significant differences in the changes in SVR between the groups. Change of SVR after administration of thiopental sodium did not have statistical significance(P< 0.05).