Part of the carbonic anhydrase activity of hepatocytes has been reported to be located in the plasma membrane. This strategic location suggests a physiological role other than that located within the cell and probably related to the specific secretory function of these cells. Furthermore, after two-thirds hepatectomy an enzymatic retrodifferentiation has been reported. We reasoned that liver regeneration probably affects the carbonic anhydrase activity in different ways depending upon its location and hence presumably physiological role. We measured, therefore, carbonic anhydrase activity in a soluble fraction or in a plasma membrane-enriched fraction obtained from liver homogenate from rats undergoing hepatectomy (two-thirds) one, three or seven days before liver resection and homogenation. No changes in carbonic anhydrase activity were found as far as soluble fraction was concerned. However, the carbonic anhydrase activity in plasma membrane was reduced (by 55%) soon after hepatectomy, there after it increased, returning to near control value at seven days. Lactate dehydrogenase activities in soluble and plasma membrane fractions were not modified by the regenerative process. Neither was 5'-nucleotidase activity determined in plasma membrane affected by liver regeneration. In summary, these results indicate a higher sensitivity of plasma membrane carbonic anhydrase activity to the regenerative process than soluble carbonic anhydrase activity. This suggests a different control of the turnover of these isoenzymes during rat liver regeneration. The phenomenon is consistent with a different physiological role for these activities; i.e., one (plasma membrane-bound carbonic anhydrase activity) may be involved in specific functions of differentiated hepatocytes, and another (soluble carbonic anhydrase activity) may be involved in general functions shared by both differentiated and undifferentiated cells.