The retina and optic nerve in the cat were examined by electron microscopy at five different time periods (1-, 3-, 8-, 16- and 24 hr) following the injection of horseradish peroxidase (HRP) into the vitreous cavity. Intra- and intercellular HRP was found in ganglion cells, Müller cells and horizontal cells even at 1 hr. Intra-axonal HRP in the retina and the retrolaminar optic nerve was seen most frequently at 3- and 8 hr. At 16- and 24 hr, many phagocytes containing HRP were noted in the nerve-fibre layer; subsequently HRP activity decreased throughout the retina. On the other hand, HRP activity in the intracanalicular optic nerve was significantly increased at 16 hr. Intra-axonal HRP appeared to be morphologically distributed in two forms: (i) HRP granules bound by a single-unit membrane mostly 0.1-0.4 micron in diameter; (ii) granules less than 0.05 micron which were occasionally hard to distinguish from artefacts such as stain deposit. The rate of orthograde axonal transport of HRP granules larger than 0.05 micron was examined on the basis of the frequency of HRP granules appearing in the retrolaminar and intracanalicular parts of the optic nerves. Though the rate seemed to be widely variable, the major velocity was calculated to be 45 mm per day, which is equal to intermediate axonal flow rates.