The effects of stimulating the reticular formation were studied during fictive locomotion in lampreys (Ichthyomyzon unicuspis). The in vitro isolated preparation of the brainstem and spinal cord was used and fictive locomotion was induced by bath application of N-methyl-D-aspartate (NMDA; 50-100 microM). During different phases of the locomotor cycle, short trains of stimuli (10 pulses at 80-100 Hz; 10 microA) were delivered through glass-coated tungsten microelectrodes positioned within the middle rhombencephalic reticular nucleus (MRRN) and their effects were studied on ipsi- and contralateral ventral root locomotor discharges. Irrespective of the locomotor phase during which the stimulation train was delivered, a resetting effect occurred. It was characterized by a re-synchronization of the locomotor discharges with a constant latency for each ventral root on the ipsilateral side. The latency increased as the recorded root was located further caudally. This increase in latency was in the range of the phase lag observed between roots during control bouts of locomotion. These results suggest that reticulospinal neurones exert strong resetting effects on spinal locomotor networks. These effects may play a significant role with respect to changes of direction during swimming.