Neuropilin (previously referred to as A5) is a neuronal cell surface protein which is widely distributed in such vertebrate species as Xenopus, chicken and mouse. In these vertebrate species, neuropilin is expressed in particular neuron circuits at particular developmental stages when axonal growth is active. To test whether the expression patterns of neuropilin is associated with axonal growth, we examined expression patterns of neuropilin in developing and regenerating Xenopus optic nerves. In embryos, a neuropilin-specific monoclonal antibody A5 (MAbA5) bound strongly to the optic nerves, and in situ hybridization signals for neuropilin mRNA were prominent in the retinal ganglion cells (RGCs), indicating that developing optic nerve fibers express neuropilin. The binding of MAbA5 to the optic nerve fibers was maximal at stages 41-43, then decreased in the subsequent tadpole life. In tadpoles after stage 50, the binding of MAbA5 to the optic nerves was extremely weak or almost nil, although RGCs expressed considerable amounts of neuropilin mRNA. When the tadpole optic nerves were crushed and prompted to regenerate, however, neuropilin protein reappeared in the optic nerve fibers. The binding of MAbA5 to the regenerating optic nerve fibers was detectable as early as the 5th day and maximal at the 2nd or 3rd weeks after the optic nerve crush, and declined thereafter. These findings suggest that the expression of neuropilin in the optic nerve fibers is regulated in an axonal growth-associated manner.