The weight of the mouse liver following feeding of ammonium perfluorooctanoate, ammonium perfluorononanoate, Telomer B ammonium sulfate, and WG-III was increased in a dose-dependent manner. Dietary levels of 3 ppm or greater ammonium perfluorooctanoate for either 14 or 21 days produced a significant elevation in liver weight both on an absolute and on an organ/body weight ratio basis. Similarly, ammonium perfluorononanoate produced significant increases at the lowest level tested, 3 ppm. Telomer B ammonium sulfate and WG-III also produced liver weight increases but at higher feeding levels. The striking increase in liver weight following relatively short-term exposures in mice makes this a useful screening test for comparing the liver-enlarging capacity of ammonium perfluorooctanoate and related fluorochemicals.