Male and female F344 rats (30-33 rats/group) were administered piperonyl butoxide (alpha-[2-(2-butoxyethoxy)ethoxy]-4,5-methylenedioxy-2-propylto luene) in the diet at levels of 0 (control), 0.6, 1.2, and 2.4% for nearly 2 years. Beginning at about 40 weeks, 10 rats in the 1.2% treated male group died due to cecal hemorrhages. Piperonyl butoxide induced hepatocellular carcinoma in both sexes in a dose-dependent manner. Hepatocellular carcinoma was found even in the 1.2% treated male group (incidence, 26.7%), and incidences in the 2.4% groups of males and females were 80.0 and 57.7% respectively of all those surviving. Piperonyl butoxide also caused essential thrombocythemia with a dose-response relationship. Hemorrhages in stomach and cecum, anemia, degenerative lesions of alveoli, and nephrotoxicity were also observed related to exposure. These results indicate that piperonyl butoxide is a hepatocarcinogen to the rat.