In rats, as in other mammals, one of the principal projections of the superior colliculus (SC) crosses the midline in the dorsal tegmental decussation to join the contralateral predorsal bundle (PDB). The cells of origin of this pathway were studied by injections of retrograde tracers (true blue or wheatgerm agglutinin-conjugated horseradish peroxidase) into the PDB rostral to its major fields of termination. Labelled cells were plotted with respect to fibre layers within the SC. The majority of labelled cells in the contralateral SC were located within the stratum album intermediale (SAI), between the fasciles oriented caudorostrally in that layer. They were of a wide range of sizes, and were much more numerous in lateral SC (where the SAI is broader) than in medial SC. The remaining labelled cells were found mainly in the deep layers. The effects of midline knife-cuts made prior to the injection of tracer suggested that most of the labelled SAI cells, but few of the deep-layer cells, projected across the midline in the dorsal tegmental decussation. Double-labelling procedures were used to investigate whether the cells of origin of the PDB send collaterals in the ipsilateral descending pathway of the SC. Injections of diamidino-yellow into the terminal regions of this pathway labelled very few PDB cells, although large numbers of other tectal cells were labelled. In contrast, extensive double labelling was observed after control injections of diamidino-yellow into the ipsilateral ascending pathway in the ventral diencephalon. These findings suggest that there is anatomical, and therefore perhaps functional, segregation of descending output channels from the superior colliculus in rat. This suggestion receives some support from comparison of the present results with those of tectal stimulation studies. Stimulation in the vicinity of lateral SAI gives contralaterally-directed head and body movements characteristic of orienting and approach, whereas stimulation in other tectal regions that contain predominantly the cells of origin of the ipsilateral descending pathway can give movements resembling avoidance or escape.