With lightly anesthetized rats, field responses to single optic nerve (ON) and optic chiasm shocks were studied in superficial layers of the superior colliculus, i.e., St. zonale (SZ), St. griseum superficiale (SGS) and St. opticum (SO). Based upon characteristics in the depth profile of the field response within the superficial layers, we identified three zones of N3, N2, and Ni from the SZ in this order. The narrow N2-zone was localized around the middle part of the SGS and less distinct than the other two zones. The N3-zone was characterized by a N3-wave and the N1-zone with Ni-and P3-waves. N3-and P3-waves were activated by slowly conducting ON fibers (5.0+1.4 and 4.2+0.6 m/sec, respectively) while the N1-wave was by fastest conducting fibers (16.3+4.1 m/sec). According to recent Golgi studies, hypothetical mechanisms underlying the three waves were proposed: the N3-wave was ascribed to the activities of small vertical fusiform cells in the SZ or the upper half of SGS, the N1-wave to those of narrow field vertical cells which predominate in the lower half of SGS, and the P3-wave to those of wide field vertical cells whose somata are in the lower half of SGS or in the SO. The N3-, Ni-, and P3-waves were found to differ from each other in recovery function tested with double ON shocks as well as in the topographical analysis of amplitudes through a whole extent of the colliculus. The three systems represented by the three waves were suggested to play different roles in visual information processing within the superficial strata.Besides the lateral geniculate nucleus (LGN) and the pretectum, the superficial layers of the superior colliculus (SC) which comprise St. zonale (SZ), St. griseum superficiale (SGS) and St. opticum (SO) are a main target of the optic nerve (ON) fibers (ALTMAN and CARPENTER, 1961;HAYHOW et al., 1962;KANASEKI and SPRAGUE, 1974;LUND, 1972). Although behavioral studies have established that the SC plays an essential part in visually guided behavior (SPRAGUE and MEIKLE, 1965), orienting behavior (BARNES et al., 1970;GOODALE and MURISON, 1975) and various kinds of eye movements (SCHILLER and KORNER, 1971;WURTZ and