1. The time-courses of the respiratory changes at the start of exercise were assesse in twenty middle-aged normal men and in twenty patients with chronic obstructive bronchitis with a moderate reduction in forced expiratory volume in 1 s (FEV1) (group M: mean 1.451; range 1.0–2.2 1) and in twenty patients with a more severe reduction in FEV1 (group S: mean 0.62 1; range 0.2–0.9 1). All subjects exercised at a constant work rate for 6 min, starting abruptly from a steady resting state. Cardiac frequency (fH), ventilation (V̇E), oxygen uptake (V̇o2) and carbon dioxide output (Vco2) were measured at the end of each minute and the rates of adaptation of these variables determined.

2. When the normal middle-aged men exercised at a low work rate [mean V̇o2 = 44.7 mmol (1.0 1 STPD) min−1] the increases by the end of the first minute were in the order: f H > V̇E > V̇o2 > V̇co2. At a work rate of 50% of their working capacity [V̇o2 = 67.1 mmol (1.5 1 STPD) min−1] the pattern of adaptation in the normal subjects was fH > V̇o2 > V̇E > V̇co2. In the patients working at 50% of their working capacity [V̇o2 approximately 44.6 mmol (1.01 STPD) min−1], the rates of adaptation were in the order: fH > V̇o2 >V̇E > V̇co2, but all adapted significantly more slowly than in the normal subjects exercising at the same work rate.

3. A reduced ventilatory capacity does not appear to be associated with any special influence on the time-course of the ventilatory adaptation at the start of exercise. The rate of adaptation of V̇E at the start of exercise is (in common with that of fH, V̇o2 and V̇co2) reduced in proportion to the overall reduction in working capacity.

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