Exogenous retinoic acid (RA) administered during mouse embryogenesis can alter the pattern of the axial skeleton during two developmental periods: an early window (7 to 8.5 days post-coitum; dpc) and a late window (9.5 to 11.5 dpc). Treatment during the early window results in vertebral homeotic transformations (predominantly posteriorizations) concomitant with rostral shifts in Hox gene expression, while treatment at the later window results in similar transformations without detectable alterations in Hox gene expression patterns. Mice null for retinoic acid receptor gamma (RAR gamma) exhibit axial defects, including homeosis of several vertebrae, therefore establishing a role for this receptor in normal axial specification RAR gamma null mutants are also completely resistant to RA-induced spina bifida, which occurs in wildtype embryos treated at 8.5-9.0 dpc, suggesting that this receptor specifically transduces at least a subset of the teratogenic effects of retinoids. To further investigate the role of RAR gamma in RA-induced defects during the early and late windows of retinoid-sensitive vertebral patterning, RAR gamma heterozygotes were intercrossed, pregnant females treated with vehicle or RA at 7.3, 10.5 or 11.5 dpc and full-term fetuses assessed for skeletal defects. Relative to wildtype littermates, RAR gamma null mutants treated at 7.3 dpc were markedly resistant to RA-induced embryolethality, craniofacial malformations, and neural tube defects. Furthermore, while RAR gamma null mutants were modestly resistant to certain vertebral malformations elicited by RA treatment at 7.3, they exhibited more pronounced resistance following treatment at 10.5 and 11.5 dpc. Moreover, several of the vertebral defects inherent to the RAR gamma null phenotype were abolished by RA treatment specifically at 10.5 dpc, suggesting that RAR alpha and/or RAR beta isoforms may substitute for certain RAR gamma functions, and that RAR gamma may elicit its normal effects on vertebral morphogenesis at this developmental stage.