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Magnetic resonance images and the histology of spheno-occipital synchondrosis were examined in young monkeys in order to compare the magnetic resonance images with their histologic observations. In serial magnetics resonance images of posterior cranial base, the spheno-occipital synchondrosis showed a low signal zone with unclear boundaries, running through the posterior cranial base perpendicularly to the clivus. The zone was always interposed between nonsignal zones. These observations were the same as those in young juvenile human beings. The histologic sections also revealed that the low signal zone was really the spheno-occipital synchondrosis, which consisted of hyaline cartilage and that the nonsignal zones were bone tissues. The chondroblasts in the spheno-occipital synchondrosis were arranged bipolarly. Intense alkaline phosphatase activity was located in the areas along the bone. Tetracycline labeling was also noticed in the bone formed in the endochondral ossification. These results suggest that magnetic resonance imaging enables us to observe the spheno-occipital synchondrosis in the posterior cranial base and also to elucidate its influences on the growth of maxilla and mandible in the future. (Am J Orthod Dentofacial Orthop 1999;115:138-42)