The development of the centrifugal projection to the chick retina in the isthmo-optic tract (IOT) has been studied by the retrograde transport of the enzyme marker horseradish peroxidase (HRP) injected into the eye at various times during the incubation period. Some neurons in the isthmo-optic nucleus (ION--the cells of origin of the IOT) can first be labeled from the eye following injections on the tenth day of incubation; after injections on the twelfth day or later, about 95% of the neurons can be so labeled. It follows from this that the axons of virtually all the neurons in the ION (including the 60% which normally degenerate between the thirteenth and seventeenth days of incubation) reach the contralateral eye. Since in 12-day old embryos the IOT is between six and seven millimeters in length and HRP can be identified in the perikarya of ION neurons within three and one-half hours, the rate of retrograde transport in the system must be of the order of 48 mm/day. A similar time is required for HRP to appear in the perikarya of ION neurons in post-hatched chicks in which the length of the IOT is estimated to be about 14 mm. This suggests that at some time during the latter half of the incubation period there is a significant acceleration in the rate of retrograde transport, similar to that found for anterograde axonal transport in the chick and rabbit visual systems.