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 degrees C, and ten were maintained at a normothermic central temperature (greater than 36.5 degrees 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 micrograms/kg) and atropine (20 micrograms/kg) was administered when the first twitch (T1) height spontaneously recovered to 10% of control (T1 = 10% of the pre-vecuronium 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 less than 0.05). The corresponding values for spontaneous recovery from T1 = 10% to TOF ratio greater than 75% were 37 +/- 15 and 80 +/- 24 min (P less than 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.