Résumé : A single oral dose of DMPT was given to pregnant NMRI mice on day 10 of gestation and the subsequent histological changes were studied in serial sections of affected limb buds isolated from 13-, 12- and 11-day treated embryos. Differences in abnormal skeletal patterns observed between fore- and hindlimb buds as well as between embryos from different litters provided clear evidence that the teratogen hits preferentially undifferentiated preskeletal mesoderm just before blastema formation. Absence or severe reduction of skeletal rudiments characterizes selectively the girdle and stylopod of hindlimbs and the zeugopod and autopod in forelimbs. In embryos slightly more advanced at the time of drug administration, the defects shifted in the zeugopod and distal segment of the posterior limb and in the distal segment only of the anterior limbs. In all cases, defects in the two distal segments displayed a postaxial predominance. Extensive cell death detected in the undifferentiated mesoderm of the affected limb parts of 11-day embryos similarly exhibited a postaxial predominance with the maximal damage in the ZPA territory. Together with the regular genesis of a postaxial subectodermal bleb in that area, followed by local involution of the AER, this observation strongly suggests that the teratogenic injury might involve an early impairment of ZPA and AER properties. In addition, predictive signs of hyperphalangy of digit I and distal duplication of the IId to e could be correlated with a transient reactional hyperplasia restricted to the preaxial part of the AER.