The distribution in the rat brain of a protein with properties similar to vitamin D-dependent calcium binding protein (CaBP) was measured using a radioimmunoassay developed for mammalian brain CaBP. The cerebellum contained very high levels of CaBP in agreement with reports of CaBP distribution in the chick brain. The protein was distributed in large and unequal concentrations in different regions of the brain. Microdissection of the hippocampus revealed that, within a particular region, CaBP can have a marked differential distribution, suggesting that it is found in specific cell types. The significance of this protein is discussed in relation to the important functions of calcium in the CNS with particular emphasis being placed upon the possible role of CaBP as an intraneuronal calcium ion buffering system.