An en bloc preparation of the mammalian superior colliculus in vitro has been used to study neurons of the superficial gray layer (SGS) with intracellular recording and HRP-technics. Electrophysiological data from kittens at 4-19 days of age suggest that at this stage SGS-neurons possess multiple spike trigger zones which can be activated by synaptic depolarization and are probably located on dendrites. In response to intratectal stimulation SGS-neurons generate EPSP-IPSP sequences or IPSPs. IPSPs are found in all penetrated cells as early as the 4th postnatal day. Ascending projection cells (APCs) and inter-layer cells (ILCs) have been identified based on antidromic activation and/or intracellular labeling with HRP. The extended dendritic arbor of APCs and ILNs (dorsal spread up to 10-20 microns below surface, horizontal spread up to 1100-1500 microns) enables these cells to sample visual information from a wide area of the visual field. Recurrent collaterals, in conjunction with potent inhibitory mechanisms, could contribute to the formation of receptive field properties of superficial tectal neurons. ILCs establish collateral connections with the intermediate gray layer.