Polysialylation of the neural cell adhesion molecule (NCAM) in the retina of Pleurodeles wald is up-regulated during metamorphosis. Incubation of larvae in thyroxine permanently induced polysialic acid expression in the retina in a concentration-dependent but age-independent manner within 3 days of incubation (3 × 10^-7 M). Retinoic acid had no effect. This suggests direct hormonal regulation of a post-translational modification of NCAM in the amphibian retina.