We compared the duration of action and recovery times for vecuronium in normothermic and mildly hypothermic patients. Ten patients were actively cooled to a central body temperature near 34.5° C, and ten were maintained at a normothermic central temperature (>36.5° C); temperature was measured in the distal esophagus. Vecuronium 0.1 mg/kg was administered as an intravenous (iv) bolus to all patients, and the evoked mechanical response to train-of-four stimulation was recorded. Five hypothermic and five normothermic patients were allowed to recover spontaneously. In the remaining five in each group, neostigmine (40 μg/kg) and atropine (20 μg/kg) was administered when the first twitch (T1) height spontaneously recovered to 10% of control (T1 = 10% of the prevecuronium twitch tension). Vecuronium's duration of action (from injection of drug until T1 = 10%) was 28 ± 4 and 62 ± 8 min during normothermia and hypothermia, respectively (P < 0.05). The corresponding values for spontaneous recovery from Tl = 10% to TOF ratio >75% were 37 ± 15 and 80 ± 24 min (P < 0.05), and for neostigmine-induced recovery were 10 ± 3 and 16 ± 11 min (difference not significant). We conclude that mild hypothermia increases the duration of action of and time for spontaneous recovery from vecuronium-induced neuromuscular blockade.