The role of visual function in (a) the normal development of intertectal neuronal connexions, and in (b) the production of the modified patterns of intertectal connexions that may develop in animals with an eye rotation, was examined. Normal animals were dark-reared from stage 58, before the first appearance of intertectal responses, until the terminal recording experiment in adult life. The general pattern of intertectal connexions was normal in these animals, but the organization was less precise than normal. It was concluded that visual function was required for the development or maintenance of the normal spatial precision of functional intertectal connexions. Animals with one eye rotated at stage 54 or stage 58 were reared subsequently in darkness until the terminal recording experiment. Such animals reared in a normal laboratory environment would have shown a modified pattern of intertectal connexions. The dark-reared animals did not. It was concluded that visual function was necessary for the reorganization of intertectal connexions that follows larval eye rotation.