Sinusoidal variations of CO2 were applied to anesthetized White Leghorn cockerels enclosed in a body plethysmograph. Waveforms were 2.5% mean plus 2.5% peak sine wave CO2 in air, at frequencies of 0.011 to 0.467 rad/s. Reflex variation in tidal volume and respiratory interval in response to the inputs of sinusoidally varying fractional content of inhaled CO2 for nine points within the above range of driving frequencies were recorded breath-by-breath. A least squares best-fit procedure was used to estimate mathematical input-output relationships (transfer functions) between the outputs, tidal volume (VT) and respiratory interval (1/f), and the single input, inspired CO2 fraction. For VT, a three time constant model most closely fitted the data. Two time constants, 80 s and 3.9 s, were associated with negligible delay; the third, 14 s, has a 0.35 s time delay. Dynamics of VT and interval reflex responses differed significantly suggesting that they are regulated by separate mechanisms.