An antibody raised against the recombinant Xenopus laevis Hoxb-7 protein (López and Carrasco  Mech. Dev. 36:153–164) recognizes the 30 kDa translation product of the Hoxb-7 gene in X. laevis and the cognate nuclear protein in chicken embryos. The X. laevis Hoxb-7 protein was expressed maternally and zygotically. Treatment of X. laevis and chicken embryos with either all-trans retinoic acid (RA) or the retinoid antagonist Ro 41–5253 (Ro; Apfel et al.  Proc. Natl. Acad. Sci. U.S.A. 89:7129–7133) during early development induced malformations of the neural tube and complementary changes in the expression domain of the homeoprotein Hoxb-7. Treatment of X. laevis embryos with retinoic acid during gastrulation induced an anterior shift of the Hoxb-7 expression domain and was correlated with an enlargement of rhombomere r7. In addition to a reduction in rhombomere numbers and of forebrain size, various malformations involving all three germ layers were observed. Treatment of X. laevis embryos with the antagonist Ro before or during gastrulation caused a progressive reduction of the Hoxb-7 domain and also dose-dependant malformations of all three germ layers. RA or Ro treatment of chicken embryos from the beginning of gastrulation caused changes of the Hoxb-7 expression domain very similar to those observed in X. laevis. In particular, either a dose-dependant loss of the Hoxb-7 protein in the neural tube or an ectopic expressionin the forebrain region was observed. The results of this study indicate that endogenous retinoids regulate the spatial expression of homeobox-containing genes in vertebrates. © 1995 wiley-Liss, Inc.