Methylene blue trihydrate has a variety of biomedical and biologically therapeutic applications. Groups of 50 male and 50 female rats and mice were administered methylene blue trihydrate in 0.5% aqueous methylcellulose solution by gavage at doses of 0, 5, 25, or 50mg/kg bw/day (rats) or 0, 2.5, 12.5, and 25mg/kg bw/day (mice), 5 days per week for 2 years. In rats survival of all dosed groups was similar to that of the vehicle controls, whereas mice exhibited a dose-dependent increase in survival. Rats receiving 25 and 50mg/kg bw/day and mice receiving 25mg/kg bw/day developed mild anemia. The incidences of pancreatic islet cell adenoma and adenoma or carcinoma (combined) were increased in all dosed groups of male rats, but increases were statistically significant in 25mg/kg bw/day males only and the dose-response was non-linear. There was a corresponding increase in the incidence of pancreatic islet cell hyperplasia but statistically significant only in the 50mg/kg bw/day male rats. There were no significant increases in neoplastic transformation observed in the mice; however, positive trends were noted for adenoma or carcinoma (combined) of the small intestine and malignant lymphoma.