We have modified the rebreathing method to study CO2 responsivity in very small mammals. Tidal volume (VT) and frequency (f) of pentobarbital-anesthetized mice were measured during rebreathing from a closed circuit, primed with 95% O2, 5% CO2, through which the gas was constantly circulated at 0.5 l X min-1. The circuit consisted of T-tube from a plethysmograph, Tygon tubing with compliant element, CO2 analyzer and pump, in series. Circuit PCO2 (PctCO2), which was recorded continuously during spontaneous breathing, rapidly equilibrated with end-tidal PCO2. CO2 response curves were constructed from extrapolated minute ventilation (V), VT, f and parameters of breath-to-breath timing, respectively, on PctCO2. Analyses of slopes of the response curves, change from onset of rebreathing to peak response, and PctCO2 at which the response peaked revealed that CO2 stimulates V by increasing f and VT and that this is effected by facilitation of central inspiratory-expiratory phase switching and inspiratory drive mechanisms. However, the stimulatory effect of CO2 on phase switching was not sustained, with maximal effect occurring before peak V. The advantages and facility of the modified rebreathing method make it suitable for studies of other small mammals, including neonates.