BACKGROUND: As of alpha2-agonist, clonidine reduces generalized sympathetic outflow in nervous system and also reduces acetylcholine release at cholinergic terminals presynaptically. So clonidine premedication is possibly able to decrease muscle contraction and prolong the duration of neuromuscular blockers. Therefore, the aim of our current study is to investigate the effect of oral clonidine on the duration of vecuronium. METHODS: Forty patients (ASA I or II) sheduled for elective low abdominal or extrimities operation were randomly divided into 2 groups. Clonidine group (n=20) received 5 microgram/kg oral clonidine at 90 min before operation. Control group (n=20) received nothing. Neuromuscular transmission was measured with relaxograph. After injection of vecuronium 0.1 mg/kg, we measured onset time (the time from injection of vecuronium to decrease to the 25% of baseline value, duration 1 (the time interval between injection and recovery of the first twitch to 25% of the baseline value), and duration 2 (the time interval between second injection of 0.02 mg/kg vecuronium and recovery of the first twitch to 25% of the baseline value). RESULTS: There were no statistical differences between control and clonidine group in onset time (2.6 +/- 0.6 min vs 2.7 +/- 0.5 min), duration 1 (37.5 +/- 8.9 min vs 40.3 +/- 8.6 min) and duration 2 (22.0 +/- 6.8 min vs 24.4 +/- 6.1 min). CONCLUSIONS: Five microgram/kg of oral clonidine premedication did not prolong the duration of vecuronium.