Circadian rhythms (diurnal variations) in many physiological parameters have been reported. However, there are no data on gas exchange kinetics at the onset of exercise. The purpose of this study was to establish whether there are circadian rhythms in gas exchange kinetics at the onset of exercise.
Six male subjects performed 120W exercise on a cycle ergometer for 7 min in the morning (AM; 7: 30-8: 30) and evening (PM; 16: 30-17: 30) . Rectal temperature (Tr) and mean skin temperature (T_sk) at rest were significantly higher PM than AM, the differences being 0.9±0.2°C and 0.7±0.2°C, respectively. Respiratory and circulatory parameters at rest and during exercise were not different between AM and PM. The time constants of oxygen uptake (Vo₂), carbon dioxide output (Vco₂), minute ventilation (V_E), heart rate (HR), and oxygen pulse (Vo₂/HR) showed the same results. There was no relationship between temperature parameters (Tr, T_sk) and the time constants.
It is suggested that circadian rhythms reflected by the change in body temperature do not have any effect on gas exchange kinetics at the onset of moderate bicycle exercise.

Under the condition that entrainment between breathing rate and exercise rhythm was minimized. The limitation for deciding anaerobic threshold (AT) by respiratory frequency (f) was studied. Ten healthy subjects (5 male and 5 female) have volunteered to take part in two incremental cycle exercises (male : 30 watt/2 min, 50 rpm ; female : 20 watt/2 min, 50 rpm) . The subjects were either sedentary or active and performed tests under two different condi-tions. The different conditions are explained below.
1) Condition M : Use a metronome to maintain pedalling frequency so entrainment would easily occur.
2) Condition S : Use a tachometer to maintain pedalling frequency so entrainment would not easily occur.
Oxygen uptake (VO₂) at AT were determined by two different methods. The first method was to detect the point of non-linear increase in minute ventilation (VE) and carbon dioxide output (VCO₂) and then to increase detection in the ventilatory equivalent for O₂ (VE/VO₂) without increasing the ventilatory equivalent for CO₂ (VE/VCO₂) (AT-V) . The second method was to detect inflection in f by multisegment linear regression (AT-CF) . There were no significant differences between AT-V (condition M : 26.0±6.2, condition S : 26.4±6.0 ml/kg/ min) and AT-CF (condition M : 31.6±10.2, condition S : 24.7±10.0 ml/kg/min) . A significant positive correlation between AT-V and AT-CF was observed in condition S (r=0.850, p< 0.05), but not in condition M (r=0.563, p>0.05) . The error between AT-V and AT-CF had individual variations. An error within±5% was observed in only 4 out of 10 subjects. These results suggested that even though the ability to detect AT using f is superior in condition S, f is an inadequate indicator for the AT, though the exercise entrained breathing is minimized.