Visual stimuli elicit motor responses in lampreys. These responses rely, in part, on the activation of reticulospinal (RS) neurones which constitute the main descending pathway in these early vertebrates. This study sought to identify and characterize possible mesencephalic relays for visual inputs to RS neurones of the rhombencephalon. The anatomical substrate subserving this function was investigated by iontophoretically ejecting cobalt-lysine, a retrograde tracer, in the middle rhombencephalic reticular nucleus in the in vitro isolated brainstem preparation of young adult Petromyzon marinus. Several populations of cells were retrogradely labeled in the brainstem. Of particular interest were the cell populations found on each side of rostral mesencephalon, located in the tectum and pretectum. There were, on average, 113 cells labeled contralateral to the injection site and 80 cells labeled ipsilateral to the injection site. The cells were morphologically similar on both sides, except that the contralateral group had larger cell bodies as compared to the group on the ipsilateral side. To determine whether the axons of the cells contacted reticulospinal neurones, electrophysiological experiments were carried out in which the region containing these cells was microstimulated. Large post-synaptic potentials were recorded intracellularly in RS neurones. Furthermore, microstimulation of the optic nerve on the same side as the recorded cell (i ON) evoked responses with a pattern similar to those resulting from stimulation of the optic tectum contralateral to the cell recorded (co OT), except for the longer response latencies. Local ejection of xylocaine (1% lidocaine hydrochloride) or CNQX (1 mM) onto the co OT reversibly abolished the responses evoked from stimulation of the i ON. There were no significant effects observed when the drug was ejected onto optic tectum ipsilateral to the cell. Taken together, the results from this study indicate that the crossed tectoreticular pathway is involved in relaying optic nerve inputs to RS neurones of the middle rhombencephalic reticular nucleus. Moreover, cells of origin of this pathway appear, in all respect, homologous to cells giving rise to the crossed tectobulbar pathway in other vertebrates.