BACKGROUND: It is stated frequently that patients with spinal block may be drowsy, although they may not have received any sedative drugs. Intrathecal clonidine increase the duration of sensory and motor blockades, and also has a sedative effect. Thus we conducted this study to investigate the effects of spinal anesthesia and intrathecal clonidine on propofol hypnotic requirements. METHODS: Forty-five adult patients scheduled to undergo local or spinal anesthesia were enrolled in this study. Group 1 included patients on local anesthesia, group 2 were patients on spinal anesthesia with 0.5% hyperbaric bupivacaine, and group 3 were patients on spinal anesthesia with 0.5% hyperbaric bupivacaine and 75microgram clonidine. The target controlled infusion (TCI) of propofol was started at a target concentration of 1microgram/ml. We checked the lowest BIS during 5 min observation after the effect site concentration (Ce) had been reached (1microgram/ml). The TCI of propofol was then restarted at a target concentration of 1.5microgram/ml and we checked the lowest BIS during 5 min observation after the Ce had been reached (1.5microgram/ml). We also checked the Ce when the BIS reached 80 and 70. RESULTS: The minimum BIS's at 1 microgram/ml Ceiiwere 86.9 +/- 11.3 (Group 1), 80.5 +/- 8.5 (Group 2) and 66.9 +/- 15.5 (Group 3), and the minimum BIS's at 1.5microgram/ml Ce were 76.0 +/- 13.4, 62.9 +/- 12.4, 48.5 +/- 13.7, respectively. The Ce's of propofol at BIS 80 were checked initially at 1.4 +/- 0.5microgram/ml (Group 1), 1.1 +/- 0.3microgram/ml (Group 2) and 0.8 +/- 0.3microgram/ml (Group 3). The Ce's of propofol at BIS 70 were 1.8 +/- 0.6microgram/ml, 1.4 +/- 0.3microgram/ml and 1.0 +/- 0.3microgram/ml, respectively. The Ce's of Group 2 and Group 3 at BIS 80 and BIS 70 were statistically lower than those of Group 1 (P < 0.05), and the Ce's of Group 3 at BIS 80 and BIS 70 were statistically lower than those of Group 2 (P < 0.05). CONCLUSIONS: Spinal anesthesia and intrathecal clonidine reduce the requirement of propofol for conscious sedation. The Ce of propofol for conscious sedation is 1.4-1.8microgram/ml for local anesthesia, 1.1-1.4microgram/ml for spinal anesthesia with 0.5% hyperbaric bupivacaine, and 0.6-1.0microgram/ml for spinal anesthesia with 0.5% hyperbaric bupivacaine and 75microgram clonidine.