The ultrastructure of rat kidney sections examined by TEM on 14 days. The renal tubules were circular, microvilli were evident in the lumen, and mitochondria were located around the nuclei in the control group. In the EG group, the renal tubules were thin with flattened tubular cells; the lumen of the renal tubule was expanded; microvilli were barely recognizable; crystals were present in the lumen; swollen mitochondria resembling fat droplets around the nuclei had an indistinct, discontinuous, and partly collapsed double membrane. In the EG + catechin (EG + catechin 2.5, and EG + catechin 10.0) groups, the renal tubules were circular and microvilli were detected in the lumen; the renal tubules were longer than those in the EG group, and the layer was thicker, but slightly shorter than that in the control group; the mitochondria had a regular internal structure with a continuous double membrane which was similar to the mitochondria in the control group. Scale bars, 1 μm.