Skull morphology in achondroplasic (cn/cn) mice was compared with that of normal siblings in order to determine the effects of this chondrodystrophy on skull growth, particular attention being given to dimensions reflecting growth at the synchondroses of the cranial base, the nasal septal cartilage and the condylar cartilage of the mandible. The central section of the cranial base (basicranial axis) was reduced by 25 %, the length of the viscerocranium by 18 % and the length of the condylar process by 11 %. The evidence indicates that these reductions are due to diminished growth at respectively the spheno-occipital and midsphenoidal synchondroses, the nasal septal cartilage and the condylar cartilage. The relative sizes of the reductions in cranial base, viscerocranium and condylar process suggest that the growth of synchondrotic and septal cartilages is diminished to a greater extent than that of condylar cartilage. This finding is in agreement with the observations that condylar cartilage, unlike synchondrotic and septal cartilage, grows by surface apposition and that the principal defect in cn/cn mice is a disturbance of interstitial cartilaginous growth. The posterior extension of the basicranial axis of the cn/cn mice was reduced by 14 % and the anterior extension by 2 %. The width of the cranial base was decreased by 9 % and the angle between the basicranial axis and its anterior extension was decreased by 3 %. The length of theneurocranium was reduced by 19 % in the cn/cn animals while the volume of the endocranial cavity was diminished by only 18 %. The latter reduction is less than would be expected from the cube relationship between volume and linear dimensions but is readily accounted for by the lack of reduction in the height or width of the neurocranium, the slight flattening of the cranial base and the doming of the neurocranial vault.