The authors studied the pharmacodynamics of four nondepolarising relaxants, d-tubocurarine (dTC), pancuronium, metocurine, and gallamine, at 25 degrees C, 31 degrees C, and 37 degrees C. The rat phrenic nerve-hemidiaphragm preparation with vascular perfusion was used for these investigations. For each drug at each temperature, a dose-response curve for twitch depression was constructed. ED50 values were calculated using probit-log dose regression. dTC, metocurine, and gallamine each demonstrated a near twofold increase in ED50 at 25 degrees C compared with 37 degrees C. No such relationship was apparent with pancuronium. In addition, the slopes of the dose-response curves were analyzed for effects due to temperature or drug. Slopes were not influenced by temperature; however, the slopes for metocurine and dTC were lower than those for pancuronium and gallamine. The authors conclude that in the rat, pancuronium retains potency at hypothermia, whereas the other relaxants decrease potency. In addition, metocurine and dTC exhibit less steep dose-response curves under these experimental conditions.