In order to evaluate the influence of age on 2‐acetylaminofluorene (2‐AAF) carcinogenesis, female BALB/c mice were fed diets containing 500 ppm 2‐AAF for 6 mo, starting at 1, 7, or 13 mo of age. Croups of mice were killed immediately, 3 or 6 mo after termination of treatment, or were allowed to complete their life span. Mice corresponding to the age of each sacrifice group of the 2‐AAF‐treated mice and a life‐span group were also included as controls. The study was moved to a different animal room area after 25 wk into the study. Thus, certain treatment groups were replicated to evaluate the impact of the change in environment. Controls and the young 2‐AAF‐treatment group killed at 7 mo of age were replicated twice in the new animal room at 6‐mo intervals. Similarly, controls and the mid‐aged treatment group killed at 13 mo of age were repeated once in the new animal room. This resulted in mice of each age/treatment group being treated simultaneously in the same room and killed as soon as treatment was stopped. The main histopathologic responses to 2‐AAF were observed in the urinary bladder, liver, and mammary glands. In all three replicates urinary bladder hyperplasia was less severe when young mice were treated than when mid‐aged or old mice were treated. In contrast, there was no clear influence of age on bladder tumorigenesis, although replicate variation in this response was very great.