['Because clonidine has analgesic and sedative properties, it may influence the awakening concentration or dose of an anesthetic. To investigate the effects of oral clonidine premedication on emergence from isoflurane anesthesia, we studied 61 ASA physical status I or II patients undergoing superficial operations. They were randomly allocated to one of three groups according to the dose of clonidine they received: the clonidine-2.5 group (n = 21), clonidine-5 group (n = 20), and control group (n = 20) received approximately 2.5, 5, or 0 microg/kg oral clonidine, respectively, in addition to 20 mg of famotidine 90 min before general anesthesia induction. Anesthesia was induced by thiamylal 5 mg/kg, and tracheal intubation was facilitated with succinylcholine 1.5 mg/kg I.V. Anesthesia was maintained with a 1.1% end-tidal isoflurane concentration and 67% N2O in oxygen, while ventilation was controlled to maintain end-tidal CO2 tension between 33 and 38 mm Hg. After surgery, N2O was discontinued while the end-tidal isoflurane concentration was maintained at 1.1%. After confirming the end-tidal N2O concentration of 0%, isoflurane was discontinued. The end-tidal isoflurane concentration at the time when patients responded to a standardized verbal command to open their eyes was recorded as MAC-Awake. The MAC-Awake values in the clonidine-5 group (0.22% +/- 0.09% [mean +/- SD]) was significantly less than those in the clonidine-2.5 and control groups (0.28% +/- 0.07% and 0.30% +/- 0.07%, respectively, P < 0.05). The wake-up time, from discontinuance of isoflurane until arousal, was longer in the clonidine-2.5 and clonidine-5 groups than in the control group (17.3 +/- 8.0, 16.9 +/- 7.0, and 10.6 +/- 5.3 min, respectively; P < 0.05). In conclusion, oral clonidine premedication 5 microg/kg decreases the awakening concentration of isoflurane, and arousal from isoflurane anesthesia is prolonged with oral clonidine in a dose of 2.5-5 microg/kg.'] Does oral clonidine premedication reduce the awakening concentration of isoflurane?