Rat duodenum was examined for light and electron microscopical localization of carbonic anhydrase (CA) activity by the histochemical Hansson's method. The absorptive epithelial cells and some goblet cells showed a distinct reaction for CA activity in the cytoplasm, and on the striated borders and lateral cell borders. Electron microscopy confirmed that the reaction precipitates for CA activity are intimately associated with the cytoplasmic side of the cell membranes on both the lateral cell infoldings and the microvilli composing the brush borders. But the epithelial cells on the crypts were not positive except for some unidentified cells which show the distinct reaction. The duodenum was divided into four segments from the proximal to the distal part, and the homogenized mucosa scraped from each segment were examined for CA activity by Maren's physiological method. It was found that the activity decreased from the proximal to the distal segments. The most proximal segment showed the highest activity, 10.4 +/- 2.1 UA/mg of protein and 1468.7 +/- 324.8 UA/g of wet weight, and the most distal segment of duodenum showed the lowest activity, 2.4 +/- 0.7 UA/mg of protein and 291.7 +/- 85.8 UA/g of wet weight. The significance of the localization of CA activity in the villous epithelium of rat duodenum was discussed in relation to the bicarbonate section.