Aroclor 1254 was fed to groups of 24 male and 24 female F344 rats, from 7 weeks of age, at dietary concentrations of 25, 50 and 100 ppm for up to 105 weeks. There was a dose- related depression of body weight gain for both sexes and decrease in survival for male rats. Histologically, an increased incidence of gastric intestinal metaplasia and adenocarcinoma was found in both sexes. Hepatocellular adenomas, carcinomas, and eosinophilic and vacuolated hepatocellular foci were usually found in dosed rats only. The number of these foci per unit area of liver was significantly increased in dosed rats, although eosinophilic foci were only found in rats exposed to Aroclor 1254. Basophilic hepatocellular foci were found in similar numbers per square centimeter of liver in controls and treated rats. This finding suggested that eosinophilic hepatocellular foci and tumors arose de novo rather than from the naturally occurring basophilic foci. The appearance of unique, potentially preneoplastic lesions and tumors in the liver and stomach in dosed rats which do not usually occur spontaneously in control rats would support the hypothesis that Aroclor 1254 induced or initiated these unique lesions de novo rather than promoted the growth of any naturally occurring lesions. Nonneoplastic hepatic lesions included degenerative hepatocellular changes and aggregates of macrophages with crystalline cytoplasmic structures and pigment granules.