Carbonic anhydrase (CA) IV facilitates renal acidification by catalyzing the dehydration of luminal H2CO3. CA IV is expressed in proximal tubules, medullary collecting ducts, and A-intercalated cells of the mature rabbit kidney (Schwartz GJ, Kittelberger AM, Barnhart DA, and Vijayakumar S. Am J Physiol 278: F894–F904, 2000). In view of the maturation of HCO[Formula: see text] transport in the proximal tubule and collecting duct, the ontogeny of CA IV expression was examined. During the first 2 wk, CA IV mRNA was expressed in maturing cortex and medulla at ∼20% of adult levels. The maturational increase was gradual in cortex over 3–5 wk of age but surged in the medulla, so that mRNA levels appeared higher than those in the adult medulla. In situ hybridization showed very little CA IV mRNA at 5 days, with increases in deep cortex and medullary collecting ducts by 21 days. Expression of CA IV protein in the cortex and medulla was minimal at 3 days of age but then apparent in the juxtamedullary region, A-intercalated cells and medullary collecting ducts by 18 days; there was little labeling of the proximal straight tubules of the medullary rays. Thus CA IV expression may be regulated to accommodate the maturational increase in HCO[Formula: see text] absorption in the proximal tubule. In the medullary collecting duct, there is a more robust maturation of CA IV mRNA and protein, commensurate with the high rate of HCO[Formula: see text] absorption in the neonatal segment.