The effect of exercise intensity on gas exchange kinetics was investigated during exercise and recovery, as well as the relationship between the kinetics during exercise and recovery. Twenty-three patients with a history of anterior myocardial infarction performed low-intensity (38.7±8.3 W) and high-intensity (68.8±15.0 W) exercise for 6 min. The time constants of oxygen uptake (VO₂), carbon dioxide output (VCO₂) and minute ventilation (VE) were significantly prolonged during high intensity exercise compared with low-intensity exercise (61.2 ±8.6 vs 52.3±10.3 s, p<0.005 for the time constant of VO₂). The time constant of VO₂ was similar during exercise and during recovery from exercise of high (61.2±8.6 vs 66.2±12.2 s) as well as low intensity (52.3±10.3 vs 55.0±10.1 s). However, the time constants of VCO₂ and heart rate were significantly shorter during recovery than during exercise. The time constants of VCO₂ and VE were significantly longer than that of VO₂ during both exercise and recovery. In the present study, it was found that (1) the gas exchange kinetics were influenced by the intensity of exercise; (2) the kinetics during recovery did not necessarily reflect the kinetics during exercise except for VO₂; and (3) the kinetics of VCO₂ and VE were delayed as compared with the VO₂ kinetics. These characteristics should be taken into account when using gas exchange kinetics to estimate cardiopulmonary responses to exercise in patients with left ventricular dysfunction. (Jpn Circ J 1999; 63: 459 - 466)