The superior colliculus is recognized as a key structure in the initiation of orientation responses of the eyes and head to visual, auditory and somatosensory cues. It is in the deep layers of the superior colliculus that these sensory inputs converge. Also present in the deep layers are efferent neurons which project to areas of brainstem and spinal cord, and thus influence orientation of the eyes and head. These deep collicular neurons are pivotal in sensorimotor transformations and have direct access to the motoneuron pool for orienting. Their response is directly dependent upon the number, type, distribution and location of synaptic inputs upon them. Deep collicular neurons are strongly influenced by the basal ganglia. It is well established that the basal ganglia are involved in the initiation of movement, and diseases which directly affect the basal ganglia also cause eye and head movement-related disorders. The basal ganglia, in particular the substantia nigra, appear to exert a tonic inhibitory influence upon cells in the deep superior colliculus. This project directly addresses the anatomical basis for the integrative properties of large efferent cells in the deep collicular layers which are involved in the control of head and eye movements. Using combined autoradiographic, horseradish peroxidase and immunocytochemical techniques for light and electron microscopy, it is hoped to show precisely how inputs from substantia nigra teminate upon deep collicular neurons, and to establish that their inhibitory effect is mediated by the neurotransmitter gamma aminobutyric acid (GABA). It is also proposed to identify other sources of inhibitory input on large efferent collicular neurons involved in orientation.