The uniocular visual field representations on the superior colliculus (SC), as estimated from multiunit response field centres about the horizontal meridian, were compared in midpontine pretrigeminal opossums (Didelphis marsupialis aurita Wied 1826). Recordings from the rostral pole (RP) and its caudal neighbour, the direct binocular region (DBR), as defined by Rocha-Miranda et al. (1978), were distinguished by the histological control. The results showed that while the hemifield contralateral to the recording site was well represented on the DBR by both eyes, the ipsilateral hemifield was generously represented at the RP only by the contralateral eye. At the RP the ipsilateral eye usually conveyed information about the vertical meridian, bringing about an expanded representation of the central visual space. Distinct patterns of representation were also recognized on graphs which relate recording sites along the AP axis of the SC with the azimuths of response field centres. The representation of the vertical reference meridian upon this axis on an oculocentric system was estimated from the DBR data and localized in the RP, at about 500 microns from the rostral end, for the ipsilateral eye (Vo') and in the DBR, at about 800 microns for the other eye (Vo). Similarly, plots of the magnification factor against the AP collicular axis indicated different strategies of representation for each eye. At the segment between 500 and 800 microns on this axis the magnification factors of the ipsilateral eye were usually much higher than those of the other eye. Furthermore, horizontal disparities between field centres were shown to have distinct distributions along the AP axis within the RP and DBR regions.(ABSTRACT TRUNCATED AT 250 WORDS)