Early changes in rabbit limb-bud fine structure subsequent to maternal administration of either hydroxyurea (750 mg HU/kg) or methotrexate (19.2 mg MTX/kg) on gestational day 12 have been analyzed in order to identify the morphologic sites of teratogenic action. Forelimb-buds were examined at 15 minutes to 32 hours posttreatment for ultrastructural alterations. HU caused early dispersion of ribosomes, condensation of nuclei, segregation of the granular portion of the nucleoli, and eventual fragmentation of mesenchymal cells. As a consequence of the extensive mesenchymal cellular death, the intercellular space among the remaining viable cells was increased. Cellular debris from fragmented cells was seen phagocytized within cells and in the intercellular space. In contrast, MTX caused an increase in cytoplasmic lipid, angular contours of nuclei, but relatively little cellular death. The most prominent change was a disruption of the normal mesenchymal architecture characterized by greatly increased intercellular space and a consequent reduction in the number of intercellular junctions. Nearly normal cytoarchitecture was restored by 32 hours. It was concluded that the primary morphologic sites of action are indeed different. Nevertheless, MTX and HU shared a common denominator since both caused disruption of intercellular contacts and possible breakdown of intercellular communication: HU through cytolethality, and MTX via disruption of intercellular junctions. This latter phenomenon may help to explain the similarities in limb defects caused by these two drugs.