Male Syrian hamsters fed a commercial diet were given a series of 12 intratracheal instillations of 3 mg benzo (alpha) pyrene adherent to 3 mg Fe2O3 in 0.2 ml 0.15 M NaCl at weekly intervals. After the last instillation, the hamsters were randomly assigned to receive either 100, 1600, or 3300 (later reduced to 2400) mug retinyl acetate per week in divided intragastric doses. Hamsters in the 2400-mug retinyl acetate group had a significantly higher incidence of respiratory tract tumors than those in the group given 100 mug retinyl acetate per week. Liver vitamin A stores increased dramatically in the groups given 1600 and 2400 mug retinyl acetate and corresponded to the administration of retinyl acetate p.o. Serum vitamin A values were not consistently related to retinyl acetate administration or to hepatic stores of vitamin A.