A model of the control of respiration during CO2 inhalation and perfusion of the surface of the medulla with mock oerebrospinal fluid having various PcO; values is presented. Ventilatory transients in response to steps of CO2 in inspired air very closely match those of experimental data for a single central hydrogen ion sensor located approximately 280 p beneath the surface of the medulla, corresponding to a position 77 % of the way between cerebrospinal fluid PcO, and deep brain tissue PcOl on a Pc02 gradient. Tidal volume and end-tidal P,,, transients obtained by perfusion of the surface. of the medulla with mock cerebrospinal fluid having a high P co* value appear to be similar in form to those obtained from chemoreceptor denervated cats. The effect of allowing the Pc02 at the surface of the chemosensitive areas to change more rapidly than in the center of the cistemal fluid is discussed. It was found that ventilatory transients from CO2 inhalation could be matched to experimental data at any number of sensor depths depending upon the rate at which the surface P,,, was allowed to change. Sufficient data was not available to determine quantitatively the correct ventilatory transient from CSF perfusion at these various sensor depths.