Although the ipsilateral descending pathway is a major output projection of the superior colliculus, little is known of its functions. We therefore carried out two studies to investigate in rats the part of the ipsilateral projection that terminates in an area ventral to the inferior colliculus, referred to as the cuneiform nucleus. The first study, described here, used orthograde and retrograde tract-tracing techniques to locate the cells of origin and precise region of termination of the tectocuneiform pathway. The main findings were as follows. Injections of WGA-HRP into the superior colliculus gave terminal label in the cuneiform nucleus and also in surrounding structures which included central grey, the midbrain tegmentum bordering the parabigeminal nucleus, and the external nucleus of the inferior colliculus. As well as the strong ipsilateral projection, there was a much weaker contralateral one which crossed the midline in the tectal commissure. Label in the cuneiform nucleus was heaviest after injections into the medial deep layers. However, no clear evidence was found for topography within the tectocuneiform projection: cuneiform label varied in intensity rather than pattern of distribution with variation in the collicular location of the injection site. Injections of retrograde tracers into the cuneiform are a labelled large numbers of collicular cells, which were distributed mainly in the deep and intermediate grey layers. In agreement with the data from orthograde tracing, the heaviest concentration of labelled cells was found in the medial deep layers. This concentration extended into the adjacent dorsolateral part of central grey. A similar distribution of labelled cells was seen after injections into the structures next to the cuneiform nucleus that also receive a tectal projection. Comparison of this distribution with that obtained from injections into other parts of the ipsilateral projection, including dorsolateral basilar pons, suggested that the projection to the cuneiform area may arise from a distinct set of collicular output cells. The projection from the superior colliculus to the cuneiform nucleus and immediately adjacent areas may therefore be also functionally distinct, mediating a particular kind of tectally-elicited response. The lack of clear topography in the projection suggests that this response may not have precise spatial direction.(ABSTRACT TRUNCATED AT 400 WORDS)