Terephthalic acid (TPA) was observed to have induced tumors due to the physical cytotoxicity of urinary calculi in the urinary bladders of rats. In the acute toxicity study, the extrapolation to humans was evaluated hastily by observation of the urinary crystals and cytotoxicity on the bladder epithelium. We examined whether it was possible to observe urine crystals and corresponding alterations on the epithelium in the urinary bladders in the acute toxicity study of TPA. TPA at a dose level of 2,000 mg/kg body weight, were administered twice by oral gavage at a 21-hr interval and the bladder mucosal epithelium was observed at 3, 6 and 9 hr after the second administration using scanning electron microscope (SEM). As a result, micro crystals were observed in the bladder surface at 6 and 9 hr after the second administration, and small raised ridges on the bladder surface, which were considered to be the effects of cytotoxicity, were observed at 9 hr after the second administration. However, these were not observed in rats administered only 0.5% sodium carboxymethyl cellulose solution, which is a vehicle of TPA. In this study, in cases where the substances induce crystals in urine in a short period, it was suggested that the urinary crystals and the alterations on bladder epithelium could be detected using SEM in acute toxicity study.