There is indirect evidence that cardiovascular responses to apnoea result in a temporary slowing of the O₂ uptake in the lungs in exercising humans. The present study was undertaken in an attempt to determine directly to what extent this occurs, and whether the magnitude of this slowing is such that it must be the result of concomitant cardiovascular readjustments and not merely a result of an isolated apnoea-induced fall in the arterial O₂ saturation (S _aO₂). Eight men performed 120 W leg exercise and performed repeated apnoeas of 10–40 s duration. Heart rate, S _aO₂, and breath-by-breath gas exchange were determined. Pulmonary O₂ uptake fell gradually as breath-holds proceeded by [mean (SEM)] 74 (3)% of the pre-apnoea O₂ uptake. This decrease was significantly larger than could be accounted for by the fall in S _aO₂ alone [S _aO₂ fall –30 (3)%], which it is estimated would have resulted in a fall of pulmonary O₂ uptake of –54 (5)%. We conclude that cardiovascular responses to apnoea contribute significantly to reducing pulmonary O₂ uptake during apnoea in exercising men.