Abstract In this mini-review, we summarize the morphological and functional correlation of cell heterogeneity in the thick ascending limb of Henles loop. The epithelial cells in the thick ascending limb are of two morphological types, smooth surface (S) and rough surface (R) cells, classified by the morphology of the apical membrane. S cells predominate in the medullary portion, while R cells predominate in the cortical portion. S cells have low apical K+ conductance and high basolateral K+ conductance, whereas R cells have high apical and low basolateral K+ conductance. The S cell participates in K+ reabsorption, while the R cell participates in K+ secretion. Glucocorticoids act on the S cell to increase K+ reabsorption across the medullary thick ascending limb and to accumulate K+ in the renal medulla by the countercurrent multiplication system. This leads, in turn, to an increase in urinary K+ excretion by reducing K+ reabsorption from the inner medullary collecting duct. The R cell is unique in that it has specific NH4+ conductance in the basolateral membrane. It is proposed that this cell may participate in the secretion of NH4+ in the cortical thick ascending limb.