Groups of male and female inbred Leeds strain rats were fed diets containing either 0.5% or 1.0% paracetamol by weight for up to 18 months. At the 1.0% dosage level, 20% of rats of both sexes developed neoplastic nodules of the liver, a statistically significant incidence. These rats also showed gross enlargement of their livers and an increase in foci of cellular alteration, the latter also being observed in the low dosage male rats. Papillomas of the transitional epithelium of the bladder developed in all paracetamol-treated groups, and three rats bore bladder carcinomas. However, significant yields of bladder tumours were only obtained from low dosage females and high dosage males. Additionally, 20 to 25% of paracetamol-treated rats developed hyperplasia of the bladder epithelium, which was not coincident with the presence of bladder calculi. A low yield of tumours at various other sites also arose following paracetamol feeding. An electron microscope study of the livers of paracetamol-treated rats revealed ultrastructural changes in the hepatocytes that resemble those that result from exposure to a variety of known hepatocarcinogens.