Body temperature (Tb) and oxygen consumption ( V ̇ O 2 ) are important determinants of ventilation ( V ̇ e). While the circadian rhythms in Tb and V ̇ O 2 have been well described, the daily pattern of V ̇ e has not due to limitations in the available methods for measuring V ̇ e. Here we describe an adaptation of the barometric method using a chamber in which a large flow through very small restrictions was generated by the combined action of a positive pressure pump on the entrance and a negative pressure pump at the outlet. In this way the chamber effectively behaved as a closed system, despite having a high enough flow for long-term recording in freely moving, undisturbed small animals. This system was then used to test the hypothesis that V ̇ e oscillates with a circadian pattern similar to that of Tb . Measurements of tidal volume (V t) and breathing rate (f), in combination with Tb and activity by telemetry, were made in eight adult rats over 4–6 days under 12:12 light:dark conditions. Both V t, f, and thus V ̇ e, showed a circadian pattern similar to that of Tb and activity; that is, values were higher during the dark compared to the light phase. The differences in V ̇ e levels according to the time of the day suggest that mechanisms involved in the control of breathing may also have circadian patterns.