In albino mammals the pattern of connections between the eye and the brain is systematically disrupted at the optic chiasm, with a proportion of axons that should project ipsilaterally being rerouted to the contralateral hemisphere of the brain. Albino mice carry a mutation at the c-locus, which encodes the tyrosinase gene. Tyrosinase is the key enzyme in melanin synthesis. In this study we have used transgenic mice generated from an albino strain in which a functional tyrosinase transgene within a yeast artificial chromosome has been inserted. We have examined the chiasmatic pathways in these and control animals and have demonstrated that the abnormality is corrected in the tyrosinase transgenic mice. The results of this study identify the key element in this abnormality. The establishment of the transgenic model provides a unique tool with which to investigate the way in which melanin shapes this region of the developing mammalian visual system.