Recent computer microscope studies of the rabbit primary auditory cortex have revealed a significant spatial orientation in the dendrite systems of lamina III/IV pyramidal and nonpyramidal neurons. In order to investigate the relationship between the branching characteristics of cortical afferents and these target cells, the terminal zones and spatial arborizations of thalamocortical and callosal afferents to the primary auditory cortex will be examined. The laminar distribution of afferent terminals will be determined using the anterograde transport of HRP-lectin conjugates following an injection into the medial geniculate body or contralateral auditory cortex. The injection site in each case will be determined physiologically using microelectrodes and acoustic stimulation of the contralateral ear. The arborizations of single thalamocortical and callosal afferents will be anterogradely labeled following an iontophoretic injection of HRP-lectin into the physiologically determined acoustic radiations or contralateral auditory field. The arborizations of single afferents will be digitized with an image combining computer microscope and reconstructed in their entirety using computer assisted section alignment and the pursuit of truncated fibers into adjacent sections. Entire arborizations, including branch points and boutons will be examined from different perspectives in order to study their spatial geometry. A variety of spatial analyses will be performed in order to quantitatively characterize and classify auditory cortical afferents. These studies will yield new information on the radial and tangential organization of afferents to the primary auditory cortex and provide a morphological foundation for studying the functional properties of the auditory cortex such as isofrequency lines and binaural interaction columns. In addition, results of the proposed experiments will establish a firm basis for future studies of the role of thalamocortical afferent activity on the development of auditory cortical dendrite systems.