Reticulospinal neurones were studied during fictive locomotion in the lamprey brainstem-spinal cord in vitro preparation. Intracellular recordings were made from the large reticulospinal Müller cells located in the mesencephalon (M1-3), rostral (I1) and middle (B1-5) rhombencephalon, the Mauthner cells, and the V cells located within the posterior reticular nucleus during fictive locomotor activity. All cells were depolarized during motor activity on either the ipsilateral (M1-3, I1, B1-5 and V cells) or the contralateral (Mauthner cell (crossed axon] side. During locomotor activity and long-lasting burst activity, phasic membrane potential oscillations occurred well correlated to the ventral root activity.