The carcinogenic effects of inhaled cerium-144-oxide (Ce144O2) aerosols emitting low doses of beta particle radiation were examined in F344/N-rats. Animals were exposed to 144CeO2 concentrations ranging from 0.4+/-0.1 to 2.6+/-0.9 kilobecquerels per liter (kBq/l) for 12 to 52 minutes. Control rats were exposed to stable cerium- oxide for 25 minutes. Rats were sacrificed from 1 hour to 448 days following exposure. Tissue concentrations of 144Ce were determined. Histopathological examinations were conducted in the lung tissue. Compared to controls, the body weight gains of male and female rats were not affected by exposure. At each exposure level, the initial lung burdens (ILBs) per kilogram of body weight were significantly higher among female rats than among male rats. Dose rates ranged from 0.072+/-0.026 to 0.81+/-0.14 gray (Gy) per day. Cumulative beta particle doses to the lung measured 3.6+/-1.3Gy, 12+/-4.5Gy, and 37+/-5.9Gy in the low, medium, and high exposure groups, respectively. The cumulative doses to the liver and skeleton of male rats were significantly lower than those of female rats. The median survival times (MSTs) of exposed males ranged from 591 to 596 days, whereas those of exposed females varied from 720 to 757 days. The MSTs of either males or females did not differ significantly from controls. Significant dose dependent increases in the incidence of pulmonary fibrosis and alveolar epithelial hyperplasia were observed. The incidence of benign lung neoplasms ranged from 0.095% in the control group to 2.4% in the medium exposure group. The incidence of malignant lung neoplasms, such as adenocarcinomas, squamous cell carcinomas, and adenosquamous carcinomas, increased significantly in a dose dependent manner from 0.48% in controls to 17% in the high exposure group. The authors conclude that the risk of lung neoplasms in rats does not increase with decreasing mean beta particle dose over the range of 3.6 to 37Gy.