Sources of afferent projections to the superior colliculus (SC) of the rat were determined by means of the retrograde horseradish peroxidase (HRP) method. The experiments were conducted first to provide a comprehensive map of all the brain structures which supply input to the SC, and secondly to pinpoint the sources of afferents to the deep collicular layers. Experiments conducted after large HRP deposits invading almost all the collicular layers resulted in the labeling of visual centers (cortical areas 17, 18 and 18a, ventral lateral geniculate nucleus, nucleus of the posterior commissure, nucleus of the optic tract, anterior and olivary pretectal nuclei, parabigeminal nucleus); somatosensory centers (cortical area SmI, principal and spinal tract trigeminal nuclei) auditory centers (auditory cortex, inferior colliculus and nuclei of the lateral lemniscus) and various other centers (zona incerta, substantia nigra, cingulate and motor cortices, and some hypothalamic, thalamic, pontine reticular and deep cerebellar nuclei). Deposits limited to the deep SC layers resulted in the labeling of a smaller number of structures: visual centers (cortical area 18a, nucleus of the posterior commissure, parabigeminal nucleus); somatosensory centers (cortical area SmI, principal and spinal tract trigeminal nuclei); auditory centers (inferior colliculus, nuclei of the lateral lemniscus); and various other centers (zona incerta, substantia nigra, cingulate cortex, some hypothalamic nuclei, posterior thalamic nucleus, central gray, cuneiformis and subcuneiformis nuclei, pontine reticular nucleus pars oralis). The pattern of the afferent connections to the rat's SC appears identical to the general plan of organization in mammals; the sources of afferents are extensive and allow the SC to be influenced by multisensory, limbic and even motor-related systems.