The ultrastructural change in rat kidneys was analyzed after intraperitoneal injection of oxalic acid. This study demonstrates fundamental alterations in the epithelial cells of the proximal convoluted tubules (PCT). At 8 h (group IV), cytoplasmic prolongations (CP) begin to be observed, which fill the lumens of PCT at 20 h (group VII) and 24 h (group VIII). Unusual mechanism of cytoplasmic secretion is suggested to this fact. Calcium salts were found on CP and in group VIII. At 16 h (group IV), intracitoplasmic membranous structures (MS) appear, which can emigrate across the microvilli into the lumens of PCT. At 24 h (group VIII), MS are also shown in basal zone and in basement membrane. We considered that CP, which can fragment, and MS are the ultrastructural substratum of the matrix of the calculis, corresponding probably to the substance A.