Changes in steady state minute ventilation, tidal volume and frequency were determined in unanesthetized lambs before and after bilateral cervical vagotomy while breathing 3, 6, and 9 % CO2 in 20 % O2 balance N2. The slope of ▵VE/▵PaCO2 fell from 0.068 to 0.033 while frequency response depended on intact vagi. Electrical stimulation of the cut proximal vagus (after bilateral vagotomy) restored CO2 responsiveness to control values. From these observations, it may be concluded that the afferent vagus is involved in the control of ventilation during hypercapnia and helps to modulate ventilatory frequency by inhibiting tidal volume.