A study of neuronal responses of frog tectum to diffuse light and a moving visual stimulus, in which paired microelectrodes were used, disclosed the excitation-inhibition constructions originating in the midbrain tectum in response to various forms of visual stimulation. It was demonstrated that the dimensions of the excitation zones of these constructions are 100-200 mu in cross section, and are separated by inhibition zones. The dimensions of the excitatory tectal constructions resulting from various kinds of visual stimulation correlated with the width of the branching zones and locations of terminals of the afferents and intratectal fibers.