The threshold of the central chemoreceptors was determined under resting and three exercise conditions in four volunteers. 2. The method used was the hyperoxic, Read rebreathing technique with prior hyperventilation. Plots of resting ventilation vs. carbon dioxide consisted of two straight-line segments of different slopes above and below a breakpoint which was taken as the central-chemoreceptor threshold at rest. 3. The threshold during exercise was determined from plots of exercise ventilation vs. carbon dioxide in a similar way, but the points for these plots were obtained in a different manner. They were obtained from a number of separate rebreathing experiments, so as to avoid the divergence between mouth and central-chemoreceptor carbon dioxide levels during rebreathing in exercise. 4. Exercise was started abruptly during rebreathing experiments similar to those at rest, at a particular level of carbon dioxide, and exercise ventilation was measured at the third breath. Each rebreathing experiment therefore provides one point for the exercise ventilation vs. carbon dioxide plot. 5. The results showed that the central-chemoreceptor threshold during exercise was not different from the resting threshold, and that the initial, fast component of exercise ventilation was independent of the chemical drive to breathe.
