This study was performed to clarify the mechanism and extent of mucosal injury following button battery ingestion. After surgical implantation of several commercially available button batteries in the stomach or appendix of rats, battery voltage, mucosal pH, and histopathologic change were assessed at eight, 16, 24, and 48 hours. Prior to leakage of the alkali from the battery cell there was a reduction in battery voltage, a rise in mucosal pH, and often ulceration or perforation in the digestive tract due to discharge (electrolytic reaction) from the battery. The extent of mucosal injury varied with battery type. Perforation and death were seen in some animals. The extent of mucosal injury was similar in fasted and nonfasted animals. Electrolytic reactions and mucosal injury occurred in both acidic and nonacidic gastrointestinal sites.