The growth of the bone ends articulating through the sphenooccipital synchondrosis in the rat occurs differentially. Considerably more bone is during growth formed in the pharyngeal part of the anterior end of the basioccipital bone than in the cerebral part. The same conditions hold good for the posterior end of the basisphenoid bone, although to a less degree. These conclusions have been reached by studying the distribution of alkaline phosphatase reaction in the perichondrium covering the sphenooccipital synchondrosis. Extension of reaction to this enzyme in perichondria has previously been shown to be a measure of growth rate. Sections of the sphenooccipital synchondrosis from 10 rats 10 days ago, incubated for reactions to alkaline phosphatase have formed the basis of the study. Quantitative expressions of the extension of enzymatic reaction have been collected. Statistical treatment of these data have resulted in the above mentioned conclusions.