The projection from the retina to the controlateral superior colliculus was studied light and electron microscopically by means of anterogradely transported horseradish peroxidase and tetramethylbenzidine histochemistry as well as light microscopically by experimental degeneration and [3H]-leucine autoradiography. Labeled boutons were found in stratum zonale (SZ) and in stratum griseum superficiale (SGS), but not in stratum opticum (S0). The number of boutons was maximal in a narrow zone in SZ about 25 to 100 micron below the surface. The labeled boutons contained numerous round vesicles and predominantly pale mitochondria. They usually formed asymmetrical synapses and contacted dendrites or boutons. Occasionally, labeled boutons were observed whose cytological features were different from those generally associated with retinotectal axons. In general, labeled boutons in SZ contained fewer mitochondria than those from SGS. Labeled myelinated axons were found throughout SGS, in the lowest part of SZ, and in SO. In upper and middle SGS they were small while in lower SGS and SO also large fibers were found.