The expression of carbonic anhydrase (CA) in the quail metanephros was investigated during embryonic development. The immunohistochemical localisation of the isoenzymes CAII and CAIII was compared with the distribution of enzyme activity visualised by a histochemical cobalt-precipitation procedure. The developmental profile of CA activity was also evaluated by means of a biochemical method. The occurrence of a moderate and diffuse CAII immunostaining from the first developmental appearance of the metanephros anlage testified to an early expression of carbonic anhydrase. This finding is discussed in relation to the involvement of the enzyme in the morphogenetic mechanisms leading to the establishment both of cell polarity and epithelial phenotype. CA expression in the renal sites that are positive in adults proved to be developmentally regulated. In the collecting duct system, enzyme activity could not be identified until the time of hatching. No CA was detected at any stage examined at the sites where, in adults, enzyme occurrence has previously been interpreted as a membrane-associated CA isoform. The differentiating renal tubules displayed no CAIII immunoreactivity. It can be argued that the bulk of the enzyme activity in the embryonic metanephros is due to the cytosolic isoenzyme CAII.