In the embryonic zebrafish retina, as in other vertebrates, retinoic acid is synthesized from retinaldehyde by two different dehydrogenases, one localized dorsally, the other primarily ventrally. Early in eye development only the ventral enzyme is present. Citral competitively inhibits the ventral enzyme in vitro and decreases the production of retinoic acid in the ventral retina in vivo. Treatment of neurula-stage zebrafish embryos with citral during the formation of the eye primordia results in eyes lacking a ventral retina. This defect can be partially rescued by retinoic acid. The results demonstrate that synthesis of retinoic acid can be selectively inhibited in vivo and suggest that retinoic acid is necessary for the proper development of the ventral retina.