The ventilatory response of six subjects to increasing levels of alveolar CO2 was measured at rest and during the hyperventilation induced by passive vibratory movements of the whole body. During vibration, addition of CO2 to the inspired air produced no increase in ventilation until the PaCOCO2 reached a critical level which coincided closely with the intersection of the vibration with the resting CO2 response curve. Above this level the vibration curve was almost superimposed on the resting one. There was no evidence of an additive effect of the two stimuli, and no increase in sensitivity to CO2 during vibration. In these respects the situation differs from that in which CO2 is combined with hypoxia or hyperthermia. The findings indicate that when respiration is stimulated by vibration and CO2 simultaneously, the resultant ventilation at any point is solely that produced by the stronger of the two stimuli. hyperventilation; PaCOCO2 and ventilation; CO2 response curve and vibration Submitted on July 30, 1964