It has been known that morphological changes are predominant at the site of intercalated cells (IC-cells) of the rat outer medullary collecting duct (OMCD) under K depletion. The changes are characterized by an increase in microplica and the stud of the apical plasma membrane in association with a decrease in the tubulovesicular membrane compartment of the apical region of the cells. It has also been shown that significant K reabsorption and acceleration of H secretion are taking place in these cells of OMCD under the K depletion. In order to clarify whether the changes are directly related to K reabsorption or H secretion, the animal experiment was carried out under any condition. In the control group, G-I, rat was fed with standard diet of normal K content (serum K; 4.5 +/- 0.2 mEq/l), whereas K content was decreased in the K depletion group, G-II, (serum K; 2.7 +/- 0.6 mEq/l). In the acid loaded group, G-III, 4 mEq/100 g BW of 2N ammonium chloride was given every day (serum K; 4.0 +/- 0.2 mEq/l). Animals were sacrificed after 14 days and kidneys were removed for morphological and biochemical examinations. As a result, a significant cell proliferation and interstitial PAS positive granules are observed in K-depleted rats under the light microscopic study. Under the electron microscope, the changes of the intracellular ultrastructure are predominant in the IC-cells of OMCD, such as 'membrane recycling'. On the contrary, no particular changes are seen in acid loaded rats under the light microscope.(ABSTRACT TRUNCATED AT 250 WORDS)