The dwarf mice used in this study demonstrated severe growth retardation related to the 5 endochondral growth sites of the midline cranial base. The principal defect appears to be a dramatic reduction of activity as judged by comparing the histology of the dwarf mice with their normal litter mates. The principal defect appears, from the [3H]thymidine data, to be a complete cessation of mitotic activity in the proliferative zone. It is not possible, from the present study, to say whether activity in the matrixogenic zone is also reduced. The anomalous enlargement of hypertrophic cells by 80 days in dwarf mice is unexplained.