Under electrical stimulation of the optic nerve, intracellular recording and staining techniques were applied to tectal neurons of the frog. In the first part of this study, two response types in 25 cells were examined. One type (type I; 21 cells) was composed of an EPSP followed by an IPSP. The other type (type II; 4 cells) was composed of two successive EPSPs followed by an IPSP. The initial EPSP of type II response was concluded to be monosynaptic, while the other ones were assumed to be disynaptic. These responses were produced by myelinated fibers. Under stronger stimulus intensities, a pair of depolarization and hyperpolarization appeared at longer latencies. In the later part of this study, 22 cells were stained with Procion Yellow. Neurons were classified into three morphological types, i.e. vertical type (V-type; 10 cells), multipolar type (MP-type; 11 cells) and small neuron (S-neuron; one cell). All V-type neurons gave rise to type I response, while either type I or type II responses were recorded from MP-type neurons.