Lung cancer can be induced in rats, by radon daughter products, after exposure as low as 25 WLM (80 mJ.h.m-3) protracted over 4 to 6 months with a dose rate of 100 to 150 WL (2 to 3 mJ.m-3). The incidence of lung cancer is not increased and is equal to that of controls when the same cumulated dose is protracted over 18 months at 2 WL (0.042 mJ.m-3).