The distribution of the calcium binding protein calbindin D28k (CaBP) in the rat and gerbil olivocerebellar system was compared and an ultrastructural analysis of the inferior olive and cerebellum was performed in the gerbil. CaBP is present in the perikarya and dendritic arborisations of inferior olivary neurons. The olivocerebellar projection in the inferior cerebellar peduncle stains for CaBP. CaBP is also contained in the terminal branches of the olivocerebellar projection: the climbing fibres in the inner molecular layer of the cerebellum. Thus CaBP is a reliable marker for the whole olivocerebellar projection in adult rodents. Presence of the calcium binding protein CaBP in inferior olivary neurons may influence the different calcium conductances of their dendritic, somatic and terminal compartments responsible for the physiological properties of these nerve cells.