Radioactivity above background level can be detected in the contralateral visual pathway of normal rats as long as 250 days after intraocular injection of tritiated proline. Silver grains disappear first from the retina, and last from the superior colliculus. In the optic tectum and the isthmic nucleus of the frog, more than 500 days are necessary to reach background level of radioactivity, when labelled retinal fibres are undergoing Wallerian degeneration. The intensity of label in tectal laminae formed by myelinated fibres is markedly reduced 12 days after the removal of the labelled eye. In the laminae of unmyelinated fibres, the decrease in the number of silver grains is less than 50% during a 150-day degeneration period. In the isthmic nucleus radioactivity was less intense, when labelled retinal fibres were degenerating in the tectum, as compared to that in normal animals. Radioactivity in the superior colliculus decreased 3 times faster than in the tectum after removal of the labelled eye. We conclude that proline is reutilized for protein synthesis. Unmyelinated fibres degenerate slower than myelinated ones in frogs, and transneuronal transport cannot be enhanced by increased protein breakdown.