Choline acetyltransferase (CAT) was assayed in the optic tectum of 4 teleost species with different visual powers. The results showed a close relationship between the enzyme levels in the optic tectum and the development of the visual system. In the more visual species, the trout, CAT activity in the optic tectum was about 30-fold higher than in the catfish, whose visual system is much less developed. Two species with intermediate development of the visual system, the goldfish and the tench, showed intermediate levels of CAT activity. Kainic acid treatment caused a significant decrease in both CAT and acetylcholinesterase (AChE) in the goldfish optic tectum. Concomitant histological examination showed, among other effects, the disappearance of most neurons belonging to the pyramidal and fusiform type in the striatum fibrosum and griseum superficiale of the tectum. The comparative and experimental data therefore suggest that the relationship between cholinergic mechanisms and the visual function is, to a significant extent, connected with the presence of intrinsic cholinergic circuits in the optic tectum. The relevance of these findings, also in relation to the problem of the identification of the retino-tectal transmitter, is discussed.