Midazolam and propofol by continuous intravenous infusion produce excellent and easily controllable sedation as an adjunct to spinal anesthesia. The purpose of the present study is to compare the cardiovascular and respiratory effect, degree of sedation and recovery of midazolam and propofol, and then determines the mean infusion rate of both groups. Forty patients of ASA class 1 or 2 scheduled to lower extremities surgery under spinal anesthesia were classified randomly into 2 groups. Group 1 were infused with midazolam 0.1-0.2mg/kg/h and group 2 propofol 2-3mg/kg/h. The results were as follows: 1) The mean induction dose of midazolam was 0.29±0.03mg/kg/h and propofol was 5.56±0.78mg/kg/h and the mean infusion rate of midazolam was 0.09±0.02mg/kg/h and propofol was 2.13±0.41mg/kg/h, which resulted in easily controllable sedation during operation. 2) The quality of sedation was assessed as good in 19 patients but 1 patient showed excitatory movements after midazolam infusion. Good sedation was provided in 16 patients but 2 patients showed excitatory movements and 2 patients complained pain on injection after propofol infusion. 3) The mean arterial pressure was more significantly decreased in propofol than midazolam group. 4) The heart rate was more decreased than control in both groups. 5) Airway maintenance was excellent and side effects were rare. 6) Recovery, judged by ability to open the eyes and recall date of birth, was significantly more rapid after propofol than after midazolam infusion.