Whole-body autoradiographic and densitometric distribution studies were performed on rats to investigate the accumulation of [14C]dimethadione (DMO) in the pancreas. [14C]DMO was intravenously administered at a dose of 167 microCi/0.5 mg/kg. Animals were sacrificed 1, 15, 30, 60 and 180 min after administration of the radioactive compound. The compound was found to rapidly distribute in all body tissues. The distribution pattern of the compound in the pancreas was spotty or linear due to the presence of the radioactive compound in blood of intrapancreatic vessels and in pancreatic ducts at higher levels than parenchyma. The radioactivity of pancreatic ducts became more evident with time, indicating the gradual accumulation of the compound in the ducts. These findings provide morphological evidence that DMO is accumulated in the pancreas and possibly eliminated from pancreatic juice. Densitometry revealed that the levels of radioactive compound in the pancreas were virtually as high as those in the liver and kidney.