The developmental profiles of carbonic anhydrase II (CA-II) and a carbonic anhydrase related protein (CARP) were studied in rat and mouse cerebella. Enzyme histochemistry, immunohistochemistry, in situ hybridisation and Western blotting were used to study the synthesis and expression of these enzymes in cerebellar sections from age matched control, CA-II deficient and lurcher mice, the latter being characterised by Purkinje cell degeneration. Both CA-II and CARP were first found to be expressed in the Purkinje cells in the 9 day old mouse, and the immunoreactivity of both peptides increased with time. Immunohistochemistry showed more intense staining of CARP than of CA-II in Purkinje cells throughout the developmental profile of the mouse, and this was mirrored by the mRNA levels determined by in situ hybridisation. Immunohistochemistry of CA-II and CARP also demonstrated the progressive dendritic growth of the mouse and rat Purkinje cells. CA-II and CARP immunoreactivity ceased by the end of cerebellar maturation. The onset of Purkinje cell degeneration was detected at day 10 in the lurcher mouse, with concomitant marked decrease in CA-II level: however CARP expression was found to be unchanged. By postnatal day 16 neither CA-II mRNA, protein, nor activity was detectable in contrast to CARP which remained at a decreased level unit the Purkinje cells population had completely degenerated. Our findings suggest a role of CA-II in the degenerative processes of the lurcher Purkinje cells, with CARP playing an important role in the development and maturation of the cerebellar cortex.