In higher vertebrates, the formation of the body axis proceeds in a craniocaudal direction during gastrulation. Cell biological evidence suggests that mesoderm formation and specification of axial positions occur simultaneously. Exposure of gastrulating embryos to retinoic acid induces changes in axial patterns, e.g. anterior and posterior homeotic transformations of vertebrae. These morphological changes are accompanied by changes in the nonidentical, overlapping expression domains of Hox genes. In this report the influence of retinoic acid, administered at the end of and after gastrulation, on vertebral patterns is described. Anterior transformations and truncations affecting the caudal part of the vertebral column characterize animals exposed on day 8 and 9. 4 hours after retinoic acid administration on day 8 + 5 hours, Hox-1.8, Hox-1.9, and Hox-4.5 transcripts were not detected in their usual posterior expression domains, whereas transcripts of the anterior Hox-1.5 gene remained unaffected. 4 days after RA exposure on day 8 + 5 hours, Hox-1.8 expression was shifted posteriorly by an effectively low dose of RA, which induced the formation of supernumerary ribs. Hox-1.8 expression was limited to posterior, disorganized mesenchyme, bulging out neural tube, some intestinal loops and the hindlimb in truncated embryos exposed to a high dose of RA. A causal relation between the delayed activation of posterior Hox genes and anterior transformations or agenesis of vertebrae is discussed. On day 10.5 posterior transformations begin to occur in the cervical region, while later exposures again affect more caudal structures. The distribution of the transformations along the vertebral column indicates an influence of RA on migrating sclerotome cells before they are finally fixed in the cartilagenous vertebrae. The findings show that the mesodermal segments originally specified during gastrulation can be respecified in their second migratory phase, with effects spreading for a second time in a craniocaudal direction. The transformations are discussed with regard to a molecular specification of axial levels by Hox codes, defined as combinations of expressed Hox genes.