The long-term objective of our research is a better understanding of the mechanisms by which one can separately and simultaneously localize many different sounds in a natural environment. In the proposed research we will examine in detail the auditory lateralization of a pure tone in the presence of a masking noise. Although the binaural perception of simple sounds such as pure tone, broadband noise, and clicks has been investigated in detail, very little is known about how one can separately lateralize components of more complex sounds such as a tone in noise. The proposed research consists of a combination of psychoacoustical experimention and theoretical modeling of various subjective attributes of a pure tone in noise and the interaural sensitivity with which the tone may be discriminated. The experimental program is directed toward obtaining a better understanding of the characteristics and limitations of pure-tone lateralization in noise. These experimental results will be used to extend our theroy of binaural perception. This theory, which is based on a hypothetical central processor that performs specific operations on the auditory-nerve response to a give acoustical stimulus, has successfully described and predicted many subjective and objective perceptual attributes of less complex binaural sounds. The results of the proposed work should provide a major improvement in our understanding of the spatial perception of complex acoustical environments. They will also be of value in the design of sensory aids for persons with visual and auditory impairments.