The clinical duration of a bolus of vecuronium 0.1 mg.kg-1 was measured in two groups of patients between 31-55 years of age undergoing elective general surgery. Eight patients received no premedication and eight others clondine 4.0-5.5 micrograms.kg-1 orally 90 minutes before arriving at the operating room. Neuromuscular blockade was continuously quantitated by recording the acceleration response to stimulation of the ulnar nerve train-of-four. Anesthesia was induced with thiamylal 5 mg.kg-1 and maintained with 50% nitrous oxide and isoflurane (Fet 0.5%) in oxygen. Supplementary doses of fentanyl (2 micrograms.kg-1 for the control group and 1 microgram.kg-1 for the clonidine group) were given before intubation and when anesthesia was considered to be insufficient. The clinical duration of vecuronium, defined as recovery to 25% of the control first twitch in the train-of-four sequence, was significantly longer in the clonidine group (51.2 +/- 7.5 minutes compared with 40.5 +/- 5.1 minutes in the control group, P < 0.01). In the duration of blockade there were no significant differences between groups for mean SBP, DBP and HR. Patients in the clonidine group required significantly less fentanyl to maintain hemodynamic stability. We conclude that clonidine prolongs the clinical duration of vecuronium by 26.4%, most probably by a pharmacokinetic interaction.