Carbonic anhydrase activity has been studied during retina development in 2 mammalian species, guinea pig and rat, which differ for birth time and gestational period as being precocial and altricial respectively. For both species, the definitive pattern of enzyme distribution corresponds to the localization of the reaction product in the Müller glial cells at the level of nucleus, perikaryon, lateral processes, and end-feet. Only in the rat retina, staining has been observed also in some amacrine cells. The results of either in situ or extra situm investigations showed that, according to tissue maturity, in the precocial species, the definitive expression of carbonic anhydrase is reached at birth time. In the altricial species, on the contrary, maturity is very delayed and may be recognized at only the 12th d of postnatal life. Present findings confirm that carbonic anhydrase is a marker for the maturity of the retinal glial cells.