The effect of dose rate on the induction of lung cancer in Syrian hamsters by 5.3 MeV alpha particles was examined by varying the number of weekly intratracheal instillations of carrier-free 210Po. By this technique, most of the radiation dose to the lungs was delivered over intervals ranging from 10 to 120 days. Protraction of exposure over 120 days was slightly more carcinogenic at lower total lung doses (24 rad), but slightly less carcinogenic at higher doses (240 rad), than exposure limited to a 10-day interval. No synergism was observed between very low radiation exposures (2.4 rad) and simultaneously administered benzo[a]pyrene. The carcinogenic effect of a single intratracheal instillation of 210Po in isotonic saline was markedly enhanced by subsequent weekly instillations of 0.2 ml of saline alone, emphasizing the importance of noncarcinogenic secondary factors in the expression of radiation-induced lung cancer.