In the present investigation, brain stem spinal cord preparations of 0-4-day-old rats were used to determine whether inspiratory-related discharges were modulated by a central pattern generator either during baseline conditions or during conditions of increased chemical drive. Spectral analyses were carried out on pairs of nerve activities during superfusion with normal solutions (pH = 7.4) and during superfusion with acidic solutions (pH = 6.8-7.0). Autopower spectra of nerve discharges in normal pH solution revealed the presence of two peaks: one in the 2-6 Hz band and the other in the 20-39 Hz band. Peaks occurring over both frequency ranges were highly correlated as revealed by coherence spectral analysis. Acidic stimulation produced no systematic changes in spectral features, for example, shifting peaks to other frequency regions, or increasing the values of coherence. The 2-6 Hz peak is most likely due to the arrival of depolarizing inputs from the brain stem that generate a ramp of activity at recording sites. On the other hand, activity in the 20-39 Hz region represents the discharge frequency of inspiratory motoneurons. The fact that coherence is present in this latter band provides evidence for short-time scale (ms) synchronization of functionally and anatomically distinct inspiratory motoneurons by a central pattern generator.