Lamprey reticulospinal neurons are rhythmically modulated during fictive swimming. The present study examines the possibility that this modulation may originate from the spinal cord locomotor networks rather than from the brainstem. To test this, the in vitro preparation of the lamprey brainstem-spinal cord was separated into two compartments which could be exposed to different chemical environments. Locomotor activity was induced pharmacologically in the caudal spinal cord compartment and reticulospinal (RS) neurons from the posterior rhombencephalic reticular nucleus (PRRN) were recorded intracellularly in the rostral compartment containing normal lamprey Ringer. Under these conditions, the membrane potential of RS neurons showed clear rhythmic oscillations which are correlated with the ongoing locomotor activity in the caudal spinal cord bath, although no locomotor discharges were present in the ventral roots of the rostral bath. Such oscillations were not present in the absence of locomotion. These results indicate that the spinal cord locomotor networks can contribute to the rhythmic oscillations which occur in RS neurons during fictive locomotion. Moreover, the latter oscillations of membrane potential are due to both phasic excitation and Cl- -dependent inhibition in the opposite phase.