The proposal is designed to test a specific regarding the guidance of growing retinal axons in the embryonic vertebrate brain. This hypotheses states the stable positional cues are arrayed on the membranes of neuroepithelial cells and that growing axons can read these positional cues and use them to navigate to their target area. The findings which inspired the formulation of this hypothesis are that retinal axons can grow toward the tectum from a variety of different starting points in the brain suggesting a distributed guidance mechanism, and that several other plausible mechanisms of axonal guidance in this system have been clearly ruled out. One possibility which has never been definitively tested in vivo, is the idea that the directed growth of retinal fibers is due to chemotaxis of the growth cone up a diffusible gradient of a tectal attractant. Our first three experiments are geared toward distinguishing this possibility from the hypothesis of membrane-bound positional cues. We propose a series of two experiments designed to provide information of the temporal and spatial appearance of axonal guidance cues in the neuroepithelium. This will begin the characterization of the guidance cues, and will thus build in an important way on the results obtained from the first set of experiments. Retinal axons can show distinct preferences in vitro for membranes derived from different brain areas. Our next set of experiments will be to describe the topography of these preferences in a choice system in vitro. We will ask if the topography found matches what is predicted from the position cues hypothesis of axonal guidance. Finally, we will consider the possibility that positional cues generated on the endfeet of radial glial cells may be imprinted onto the basement membrane of the brain. We devise an in vivo experiment in which retinal explant are plated directed onto basement membrane. We are on the verge of understanding basic principles of neuronal pathfinding in the vertebrate brain. The experiments described here could consolidate this understanding and bring the problem to a new cellular and molecular level of analysis.