The teratogenic effect of all-trans retinoic acid (RA) was tested in the system of congenic and recombinant inbred (RI) strains of the laboratory rat carrying the mutant Lx allele which determines the polydactyly-luxate syndrome. It was demonstrated that the teratogenic effect of RA is influenced by both the genotype at the Lx locus and the modifying genes. The dose of 100 mg/kg administered by gavage on day 11 of pregnancy induced a statistically significant incidence of preaxial polydactyly in hind limbs of LEW/BN, +/Lx foetuses heterozygous in the mutant allele, whereas no specific limb defects were noted in foetuses LEW/BN, +/+ without the mutant allele. In foetuses homozygous in the mutant allele LEW/BN, Lx/Lx and SHR/BXH2, Lx/Lx, RA conversely induced a significant reduction in the number of toes on the preaxial side of hind limbs and a reduction of the zeugopodium, mostly the tibia. Reduction changes were more marked in SHR/BXH2, Lx/Lx foetuses, in which the tibia was entirely missing and sirenomelia was found. The foetuses in this group have in their genetic background a combination of modifiers that are responsible for oligodactyly and tibial hemimelia in the BXH2 strain. On the basis of continuing mapping of the rat genome, the testing of RA in the system of RI and congenic strains will be utilized for identification of so far hypothetical genes involved in morphogenesis.