In 19 cats anesthetized with chloralose and urethrane, d-tubocurarine (d-TC) was continuously infused to produce constant 90% depression of twitch tension prior to injection of 5, 10 or 20 .mu.g/kg of neostigmine. The required mean infusion rates of d-TC were 3.6 .+-. 0.6, 5.1 .+-. 0.3, 8.5 .+-. 0.3 and 8.9 .+-. 0.5 .mu.g/kg per min at body and muscle temperatures of 28, 31, 37 and 41.degree. C, respectively. The doses of neostigmine needed for 50% antagonism of the d-TC-induced depression of twitch height were 8.7, 9.2, 10.5 and 12.0 .mu.g/kg at 28, 31, 37 and 41.degree. C, respectively. The time to peak effect and duration of action of 5 and 10 .mu.g/kg of neostigmine were longer at 28 and 31.degree. C than at 37 and 41.degree. C. Although these times also were longer at 27 and 31.degree. C with 20 .mu.g/kg dose of neostigmine, they were not statistically different. Hypothermia augments a d-TC neuromuscular blockade. Hypothermia prolongs time-to-peak effect and duration of neostigmine but does not affect peak magnitude of antagonism of d-TC by neostigmine.