1. The threshold of the ventilatory response to carbon dioxide mediated by the peripheral chemoreceptors was determined under mild hypoxic conditions during both rest and exercise in eight volunteers. 2. The method used was an adaptation of the Read rebreathing technique, modified for hypoxia and with prior hyperventilation. The method produced values of ventilation and carbon dioxide which, when plotted against each other, exhibited three straight-line segments of differing slopes. The break-points were interpreted as the resting peripheral- and central-chemoreceptor thresholds. 3. Similar plots of exercise ventilation and carbon dioxide were used to determine the peripheral-chemoreceptor threshold during exercise. However, the points for these plots were obtained from a number of separate rebreathing experiments in such a way as to avoid the divergence between the carbon dioxide levels as measured at the mouth and those at the site of the central chemoreceptors, which would normally occur during rebreathing in exercise. 4. During the course of rebreathing experiments similar to those done at rest, mild treadmill exercise was begun abruptly. The ventilation measured at the third breath of exercise was plotted against the level of carbon dioxide at which the exercise started. In this way, each such rebreathing experiment provided a single point for the plot of ventilation against carbon dioxide in exercise. 5. The results showed that the threshold of the ventilatory response to carbon dioxide mediated by the peripheral chemoreceptors was approximately 39 mmHg (5.2 kPa) while that for the central chemoreceptors was approximately 45 mmHg (6.0 kPa). Neither the peripheral-chemoreceptor threshold, nor the peripheral-chemoreceptor sensitivity to carbon dioxide was changed at the start of exercise.