The effecl of total body cooling on force of contraction of the adductor pollicis was determined during a constant rate infusion of vecuronium. Anesthesia was induced with thiopental and maintained with isoflurane/nitrous oxide in eight volunteers (study group) and seven surgical patients (control group). After train-of-four (TOF) stimulation of the ulnar nerve, we measured the amplitude of the first response (Tl) in the train and the ratio of the fourth-to-first response (TOF ratio). Vecuronium was then administered as an intravenous (IV) bolus, 25 ug/kg, followed by continuous IV infusion, 25 μg-kg-1h-1, central body (core) temperature was maintained stable for 60 min, at the end of which Tl and TOF responses were constant. In the study group, core temperature was then reduced (using circulating-water blankets) by a mean of 2.6°C, decreasing the T1 and TOF ratio, respectively, by 19% and 18% per °C reduction in adductor pollicis temperature. Normothermia was maintained in the control group for a mean of 111 min, with no significant change in Tl and TOF responses. We conclude that, during a constantrate infusion of vecuronium, the magnitude of neuromuscular block increases significantly when adductor pollicis temperature decreases secondary to core cooling.