The influence of hypothermia (27℃) on the action of nondepolarizing neuromuscular blocking drugs was investigated using rat phrenic nerve-hemidiaphragm preparations. The pH of modified Kreb's solution was maintained constant by reducing the concentration of CO2 aerating the solution to 4% at 27℃ from 5% at 37℃. Log dose-response curves were constructed for pancuronium, vecuronium, pipecuronium, d-tubocurarine (d-Tc), metocurine and gallamine at 37℃ and 27℃, respectively. The slopes were not influenced by temperature. The ED(50) of pancuronium, vecuronium and pipecuronium at 27℃ were significantly smaller than that at 37℃. However, the ED50 of gallamine increased and that of d-Tc and metocurine did not change when temperature was lowered. In addition, the effects of lowering temperature (27℃) on the partial neuromuscular blockade produced by pancuronium, vecuronium and their 3-deacetyl, 17-deacetyl and 3, 17-deacetyl metabolites were studied. Neuromuscular blockade produced by pancuronium, vecuronium and their 3-deacetyl metabolites increased, while that by 17-deacetyl and 3, 17-deacetyl metabolites decreased when temperature was lowered. In conclusion, the difference in the influence of hypothermia among the neuromuscular blocking drugs, is not attributed to their steroidal structure, but may be due to the differences in their mechanism of action.