The early development of the zebrafish Brachydanio rerio is shown to be suitable as a test system for prescreening drugs suspected to be hazardous to humans. The teratogenic effects of all-trans-retinoic acid (tretinoin) and eight chemically related substances, such as 3,4-didehydro-retinol (vitamin A(2)), 4-oxo-retinoic acid and several cis isomers, are analysed. Zebrafish development offers several advantages as a test system: large amounts of eggs can be collected throughout the year. Because the embryo is translucent, organ development can be monitored in vivo. After 1 day, eyes, heart and blood circulation is visible already, and the development is synchronous up to day 3 after fertilization. The effects of the retinoids can be scored easily, and the substances can be ranked and be compared by their effectiveness. Further, a new scoring method is presented which also allows comparison of the relative strength of the teratogenic effects of the substances on the various organs enabling investigation of the structure-activity relationship. Concentrations of 10(-19)-10(-6)M all-trans-retinoic acid generate malformations such as oedema, brain deformities (anophthalmy, microcephaly and acephaly), duplication of the otic placodes and otoliths, and a shortened and bent tail. The related substances display similar effects but they differ in their effectiveness. With respect to the chemical structure, the order of potency is found to be: acid > aldehyde > alcohol; all-trans 9-cis > 13-cis; tretinoin = 3,4-didehydro > 4-oxo. The same order of potency is reported for mammals. Because of this similarity early zebrafish development appears to be a proper model system for predicting teratogenic effects of drugs in mammals, especially of chemicals and their metabolites, which are able to cross the placental barrier and reach the mammalian embryo in utero.