2-Acetylaminofluorene (2-AAF) was administered in the feed (60, 75, 100 or 150 ppm) to 3,314 BALB/c female mice for periods of either 9, 12, 15 or 24 mon. All mice were killed at 24 mon to compare the effects of continued versus discontinued administration of 2-AAF on morphologic characteristics of hepatocellular and urinary bladder neoplasms. Hepatocellular neoplasms included ratio of hepatocellular adenomas to carcinomas, degree of differentiation and incidence of pulmonary metastases. Characteristics evaluated for the bladder tumors included ratio of local to diffuse carcinomas, ratio of invasiveness to noninvasiveness of the carcinomas, ratio of types of bladder carcinoma and incidence of pulmonary metastases. Differences in the ratio of adenomas to carcinomas, degree of differentiation and incidence of pulmonary metastases were not different between the continued and discontinued group. Although the incidence of bladder carcinomas was higher in the mice on the continued 2-AAF diet, other morphologic and biologic characteristics were similar.