We examined carbonic anhydrase and carbonate content in developing dental enamel to clarify some problems in connection with the initial mineralization. Two-dimensional electrophoretic analysis showed that the type of carbonic anhydrase isoenzyme in developing enamel tissue was a low-activity form. A relatively high activity of carbonic anhydrase was detected in the developing enamel of rat lower incisors. We also determined that the content rate of carbonic anhydrase accounts for about 4% of the total matrix proteins of early developing enamel. A remarkable reduction of carbonic anhydrase activity occurred at the maturation stage, and then disappeared in the fully matured enamel. The carbonate content also decreased gradually, showing a parallel profile to that of carbonic anhydrase activity, from 3.5% to 1.9% in mineral phase. From the present study, we proposed that carbonic anhydrase in the initial site of mineralization may act as a trigger for the crystal nucleation by supplying the carbonate ions into the precursor mineral at the early stage of crystal development.