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SUMMARYThe development of axonal transport in the optic nerve of the developing chick has been investigated. Following monocular injection of radioactive fucose and proline, the specific activity of protein from paired optic lobes was determined in chick embryos and new-hatched chicks. In addition, the magnitude of the acid soluble precursor pool within lobes was assayed. The specific activity of protein in the optic lobe contralateral to (and innervated by) the injected eye relative to that of protein within the corresponding ipsilateral lobe (not innervated by the injected eye) was used as an index of axonal flow. In embryonic chicks from 8-12 days of age, four hours after injection of 3 H -f~~a~e into a single eye, this ratio was somewhat above unity. In the 13-day incubated embryo the ratio rose abruptly to over 10, and remained high throughout the period studied (up to one day after hatch). No similar sharp rise was found for the corresponding ratio for acid soluble radioactivity which remained under 2 in all ages of chicks examined. Parallel experiments with 14C-proline resulted in a smaller and more gradual development of asymmetry of specific activity of protein from paired optic lobes.These data suggest that, at the 13th day of incubation, specialized mechanisms develop for the rapid distal transport of glycoprotein synthesized within the retinal ganglion cells to terminations in the optic tectum. This phenomenon follows the major development of cerebral oxidative metabolism, but precedes the appearance of spontaneous or evoked potentials within the chick brain.