The purpose of this study was to examine the effects of reduced muscle temperature (Tm) on gas exchange kinetics and haemodynamics at the start of exercise. Six male subjects performed moderate cycle exercise under reduced (C) and normal (N) Tm conditions. Tm and rectal temperature were significantly reduced by immersion in cold water (by 6.6 degrees C and 1.8 degrees C, respectively). The increases in oxygen uptake (Vo2) and oxygen pulse (Vo2/HR) during phase 1 (abrupt increase after the start of exercise) were significantly lower under C than under N. The time constant for O2 under C (36.0 +/- 7.7 (SD)s) was significantly greater than under N (27.5 +/- 4.4 s); however, the time constants of cardiac output under C (38.3 +/- 16.6 s) and N (33.7 +/- 18.5 s) were similar. These results suggest that the slower Vo2 on-response under reduced Tm conditions is caused by decreased O2 extraction in working muscle and/or by impairment of oxidative reactions by reduced muscle temperature.