Biochemical, light and electron microscopic, and cytochemical studies were made of the proximal tubules in the kidney of adenine-ingesting rats fed on a 0.75% adenine diet for 1 to 4 days. Histologically, acicular crystals appeared in the proximal tubules on the 1st day and increased in number and size day by day. The alkaline-phosphatase (Al-Pase) activity in the kidney was increased on the 1st day and decreased after the 2nd day, but the serum Al-Pase activity remained stationary during the experiment. On the other hand, the acid-phosphatase (Ac-Pase) activity showed no variation in both the kidney and serum. Elec-tron microscopy revealed that 2, 8-dihydroxyadenine (DHOA) crystals were precipitated in the microvilli and apical region of the proximal tubular epithelia, and increased crystals were accompanied by degenerative changes in the cells of these tissues. Cytochemically, Al-Pase activity was observed in the microvilli of the proximal tubular epithelia and diminished according to loss of the microvilli caused by the formation of DHOA crystals. Ac-Pase was well preserved in the lysosomes of the proximal tubular epithelial even when DHOA crystals had grown so as to occlude the tubular lumina.