Respiration and shivering were measured in unanaesthetized, cold-exposed pigeons using pneumotachography and electromyography, respectively. The instantaneous intensity of shivering in the pectoral muscle varied in phase with respiration. Power spectral analysis showed that the main frequency components of respiration and demodulated EMG coincided exactly. The intensity of shivering was highest during end-expiration and lowest at end-inspiration. This was confirmed by cross-correlation analysis of respiration and demodulated EMG. The absolute level of modulation remained constant (c. 10 microV peak-to-peak) despite changes in the general intensity of shivering. On the other hand, the relative depth of modulation was highest during incipient shivering. These facts indicate that only a part of the motor units recruited for shivering is susceptible to respiratory modulation and that this part is first recruited during incipient shivering. Inhalation of 5% CO2 did not affect the interaction between respiration and shivering although respiration frequency varied from 25 to 60 min-1. Thus, pulmonary chemoreceptors do not mediate this effect. It is suggested that the interaction between respiration and shivering occurs directly in the CNS. The question whether the interaction is adaptive for the animal or merely reflects a common evolutionary history of the underlying neural circuits is discussed.