Objective To quantify the effects of medetomidine on the onset and duration of vecuronium-induced neuromuscular blockade in dogs. Study Design Randomized, prospective clinical study. Animals Twenty-four, healthy, client-owned dogs of different breeds, aged between 6 months and 10 years and weighing between 5.0 and 40.0 kg undergoing elective surgery. Methods Dogs were randomly allocated to two groups. Pre-anaesthetic medication in group M+ was intramuscular acepromazine (ACP) 25 mu g kg(-1), morphine 0.5 mg kg(-1) and medetomidine 5 mu g kg(-1). Group M- received ACP and morphine only, at the same dose rate. After induction with thiopental, anaesthesia was maintained with halothane in oxygen and nitrous oxide. End-tidal halothane concentration was maintained at 1.1%. Neuromuscular blockade was produced with intravenous vecuronium (50 mu g kg(-1)) and monitored using a train of four stimulus applied at the ulnar nerve. The times taken for loss and reappearance of the four evoked responses (twitches [T]) were recorded. Normal and nonparametric data were analysed with an independent t-test and Mann-Whitney's U-test, respectively. Results The fourth twitch (T4) disappeared at similar times in each group: 107 +/- 19; [72-132] (mean +/- SD; [range]) seconds in M+ and 98 +/- 17 [72-120] seconds in M- dogs. The first twitch (T1) was lost at 116 +/- 15; [96-132] seconds in group M+ and 109 +/- 19; [72-132] seconds in M-. The fourth twitch returned significantly earlier in M+ dogs: 20.8 +/- 3.8 [14-28] minutes compared with 23.8 +/- 2.7 [20-27] minutes (p = 0.032). The duration of drug effect (T4 absent) was significantly shorter (p = 0.027) in M+ (18.9 +/- 3.7 minutes) compared with M- dogs (22.2 +/- 2.9 minutes). The recovery rate (interval between reappearance of T1 and T4) was significantly more rapid (p = 0.0003) in medetomidine recipients (3.0 +/- 1.2 versus 5.2 +/- 1.3 minutes). Conclusion and clinical relevance Medetomidine 5 mu g kg(-1) as pre-anaesthetic medication shortened the duration of effect of vecuronium in halothane-anaesthetized dogs and accelerated recovery, but did not affect the onset time. These changes are of limited clinical significance.