A case is described of a 2-year-old girl who swallowed an alkaline disc battery containing mercuric oxide. Two days after ingestion it disintegrated in the stomach necessitating laparotomy to remove the battery casing and most of its contents. Postoperatively her blood mercury concentration rose to 340 micrograms/l and subsequently she developed small bowel obstruction due to adhesions. She was treated with dimercaprol but blood mercury concentrations did not fall until after a second laparotomy to relieve the obstruction and to remove residual mercury salts from the colon. The corrosive effects of swallowed disc batteries are well documented. The maximum blood concentration of mercury reported in this case is 17 times the 'acceptable level of mercury in the blood' and nearly double the highest level recorded previously after disc-battery ingestion. A policy for management of swallowed batteries is suggested.