The oral and intravenous administration of tolonium chloride (toluidine blue), a metachromatic dye, has been studied in 15 rats with alcohol-induced benign gastric ulcers and in 55 rats with malignant gastric lesions induced by 3-methylcholanthrene. Results in the series treated intravenously were unsatisfactory. Oral administration of 2 ml of 1.0% tolonium chloride gave the most effective results. Eight of 28 benign lesions were stained, but these occurred in rats killed within 12 hours of dye administration. Twelve of 13 malignant lesions were stained, with four of the lesions occurring in rats killed more than 32 hours after administration of the dye. The data suggest that oral administration of tolonium may aid in the endoscopic and surgical identification of malignant gastric lesions.