The spheno-occipital synchondroses of 20 cynomolgus monkeys have been examined histologically and radiographically. The findings by these two methods have been compared and correlated with the dental age of the monkeys. The radiographic examination was carried out on the isolated clivus region while serial sections were used for histological examination. Comparison of the synchondrosis by the two methods resulted in six disagreements when the synchondroses were classified as open, partly closed or closed. It is suggested that this difficulty is due to superimposition of shadows on the radiographs and small degrees of histological union not being recognised radiographically. It is further suggested that care should be taken in determining the age at closure when radiographs alone are used as the criterion. The dental age at initial closure appeared to vary widely in some cases occurring at an early mixed dentition stage and in others as late as the adolescent dentition. The closure of the synchondrosis was not related to the eruption time of any particular tooth. The pattern of closure did not conform to the classical type, i.e., from above downwards nor indeed was any consistent pattern revealed in either the radiographs or histological sections.