_1), and compared it to ventilation at rest (mean ± S.E.M. = 9.1 ± 0.7 l min _1 ). The difference was significant (Student's paired t test, P < 0.001). We also considered the threshold P CO 2 observed during rebreathing to be an estimate of the chemoreflex threshold at rest (mean ± S.E.M. = 42.0 ± 0.5 mmHg). However, P ET,CO 2 during rebreathing estimates mixed venous or tissue P CO 2 , whereas the resting P ET,CO 2 during resting breathing estimates P a,CO 2 (arterial partial pressure of carbon dioxide). The chemoreflex threshold measured during rebreathing was therefore reduced by the difference in P ET,CO 2 at rest and at the start of rebreathing (the plateau estimates the mixed venous P CO 2 at rest) in order to make comparisons. The corrected chemoreflex thresholds (mean ± S.E.M. = 26.0 ± 0.9 mmHg) were significantly less (paired Student's t test, P < 0.001) than the resting P ET,CO 2 values (mean ± S.E.M. = 34.3 ± 0.5 mmHg). We conclude that both the behavioural and chemoreflex drives contribute to resting ventilation.

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