BACKGROUND: This study was undertaken in surgical patients to evaluate the comparative analgesic, hemodynamic, sedative effects of clonidine or epinephrine when added to lidocaine solution in patients receiving epidural anesthesia. METHODS: The effects of clonidine were assessed in 38 men scheduled for knee surgery. Patients received 20 ml of 2% lidocaine with clonidine 50 microgram (group C50, n=11), with clonidine 100 microgram (group C100, n=10), with epinephrine 100 microgram (group E, n=9), or plane (group L, n=8) via epidural catheter. Sensory anesthetic level was assessed by pinprick and the time to L1 analgesia and the maximum number of segments of analgesia were checked. Systolic blood pressure (BP) and heart rate (HR) were measured at 2, 5, 10, 15, 20, 30, 45, 60 min after epidural injection. Sedation was defined on a scale of 0 to 3. The sedation score was checked at 5-minute intervals after epidural injection. Samples of venous blood were obtained before and 5, 10, 20, 30, 45, 60, 90 min after epidural injection. RESULTS: No significant differences in the time of L1 analgesia and number of segments of analgesia found at any observation period among the four groups of patients. There was no decrease in BP and no differences among four groups. HR decreased significantly in patients given clonidine, but did not change in those given lidocaine plain or with epinephrine. Significant differences were also observed in sedation scores between clonidine group and group L or E. Maximal plasma lidocaine concentrations in group C100 were similar to the group C50 and L, but were significantly greater than those in group E (p<0.05). CONCLUSIONS: These results indicate that the addition of clonidine to lidocaine for epidural anesthesia provides a sedative effect and relatively stable hemodynamics, and that clonidine in a concentration 1:200,000 in contrast to 1:200,000 epinephrine, tends to increase rather than to suppress the plasma lidocaine concentration.