Factors regulating the differentiated phenotype of principal cells (PC) and A- and B-intercalated cells (IC) in kidney collecting ducts are poorly understood. However, we have shown previously that carbonic anhydrase II (CAII)-deficient mice have no IC in their medullary collecting ducts, suggesting a potential role for this enzyme in determining the cellular composition of this tubule segment. We now report that the cellular profile of the collecting ducts of adult rats can be remodeled by inhibiting CA activity in rats by using osmotic pumps containing acetazolamide. The 31-kDa subunit of the vacuolar H⁺-ATPase, the sodium/hydrogen exchanger regulatory factor NHE-RF, and the anion exchanger AE1 were used to identify IC subtypes by immunofluorescence staining, while aquaporin 2 and aquaporin 4 were used to identify PC. In the cortical collecting ducts of animals treated with acetazolamide for 2 wk, the percentage of B-IC decreased significantly (18 ± 2 vs. 36 ± 4%,P < 0.01) whereas the percentage of A-IC increased (82 ± 2 vs. 64 ± 4%, P < 0.01) with no change in the percentage of total IC in the epithelium. In some treated rats, B-IC were virtually undetectable. In the inner stripe of the outer medulla, the percentage of IC increased in treated animals (48 ± 2 vs. 37 ± 3%, P < 0.05) and the percentage of PC decreased (52 ± 2 vs. 63 ± 3%,P < 0.05). Moreover, IC appeared bulkier, protruded into the lumen, and showed a significant increase in the length of their apical (20.8 ± 0.5 vs. 14.6 ± 0.4 μm,P < 0.05) and basolateral membranes (25.8 ± 0.4 vs. 23.8 ± 0.5 μm, P < 0.05) compared with control rats. In the inner medullary collecting ducts of treated animals, the number of IC in the proximal third of the papilla was reduced compared with controls (11 ± 4 vs. 40 ± 11 IC/mm², P < 0.05). These data suggest that CA activity plays an important role in determining the differentiated phenotype of medullary collecting duct epithelial cells and that the cellular profile of collecting ducts can be remodeled even in adult rats. The relative depletion of cortical B-IC and the relative increase in number and hyperplasia of A-IC in the medulla may be adaptive processes that would tend to correct or stabilize the metabolic acidosis that would otherwise ensue following systemic carbonic anhydrase inhibition.