The role of central chemosensors in the overall ventilatory response of freshwater turtles (Chrysemys scripta elegans) to the addition of CO2 in inspired gas was measured. Centrally mediated ventilatory responses were isolated in the unanesthetized animal by combining CO2 breathing and brain ventricular perfusion with mock cerebrospinal fluid (CSF) of varying acid-base status. Breathing 4.5% CO2 resulted in increases in both ventilatory frequency (f) and tidal volume (VT), with increases in VT providing most of the overall ventilatory change. Alterations in the acid-base status of the perfusate produced highly significant changes in f. VT changes were divorced from the acid-base status of the mock CSF perfusate. We therefore conclude that ventilatory changes in turtles, mediated by central chemosensors, are primarily affected by alterations in f. VT changes, associated with acid-base homeostatic mechanisms, are mediated by receptors outside the blood-brain barrier in these animals. On the basis of these data, we hypothesize that the increase in f observed when turtles breathe 4.5% CO2 is primarily mediated by the central chemosensors.