Ascorbate is found in high concentrations in the aqueous, lens and retina of a variety of animal species. Several functions have been proposed for this, including a vital role in aqueous secretion. We investigated the possible presence of ascorbate in the early embryonic chicken eye as a marker for the differentiation of the ciliary epithelium. Embryonic chicken eyes were processed for histochemical demonstration of ascorbate by silver staining. A strongly positive reaction was found in the interstitial matrix between the lens rudiment and the optic vesicle of the 2-day chicken embryo. By 3 days, with lens and optic cup separating from each other, the silver grains were concentrated in the vitreous cavity, primarily in a band-like configuration between the lens equator and the rim of the optic cup, which is the presumptive ciliary epithelium. A weaker reaction was seen in the lens vesicle and primitive neural retina. During the second and third day of incubation the optic vesicle is just beginning to expand. The appearance of ascorbic acid in the newly formed vitreous cavity at this time may indicate its importance in the biosynthesis of the matrix that is filling the expanding optic vesicle.