Teratogenic properties of thalidomide were tested in two systems of laboratory rat strains carrying the mutant lx allele that determines the polydactyly-luxate syndrome. In agreement with our previous experiments, we have confirmed that the response of foetuses basically depends on their genotype. Foetuses of LEW/BN, +/+ genotype remained unaffected following 500 or 3 x 500 mg/kg thalidomide doses (43 and 56 foetuses, respectively). In LEW/BN, +/lx foetuses these doses elicited 24% and 87% hind feet polydactyly (14/59 and 53/61 foetuses, respectively), which was highly significant when compared with 84 vehiculum-treated and 235 untreated controls (P < 0.001). However, in 48 SHR/RI 2, lx/lx foetuses both pairs of limbs were affected in an opposite way after the 500 mg/kg thalidomide dose: hind feet oligodactyly (94/96 limbs) and increased front feet polydactyly occurred (in comparison with 70 controls, P < 0.001). The mutant lx allele as well as modifying genes are involved in the response to thalidomide.