The role of the carotid chemoreceptors in the hyperpnea of exercise was investigated. The activity of the sinus nerve of the cat was monitored while the blood supply to the carotid body was controlled independently of the systemic circulation. By this technique, fluctuations in the arterial blood gases during a short interval of exercise induced by electrical stimulation of hindlimb muscles were unable to affect the chemoreceptor activity. While minute ventilation increased by an average of 51%, chemoreccptor discharge was found to be unchanged in 12 experiments, 6 while perfusing with normoxic blood and 6 while perfusing with hypoxic blood. Thus, it must be concluded that alteration of carotid chemoreceptor sensitivity does not occur during artificially induced exercise in anesthetized cats. However, the difference in the time course of ventilation following the initiation of artificially induced exercise between cats and other species does not allow it to be ruled out in other species, including man. Indirect evidence is against such a role.