The recent outbreak of renal failure in infants in China and in animals in USA and Europe has been determined to be caused by melamine adulterated in the food. In the course of the investigation, cyanuric acid was identified in addition to melamine in the offending food. Fish feeds were also recently found to be contaminated with melamine. The purpose of this study was to characterize the histopathological effect and toxicity potential of different concentrations of melamine and cyanuric acid in the kidney of Japanese catfish (Silurus asotus). The fish were administered melamine and cyanuric acid in combination at the concentrations of 12.5, 25, 50, 100 and 200 mg/kg/day for 3 days by single oral administration dissolved in carboxymethyl cellulose. The results showed that renal crystals were observed in renal tubules and collecting ducts at the concentration over 25 mg/kg dose group and the number of crystals in kidney were in proportion to the concentrations of melamine and cyanuric acid.