Groups of mice (CF/1, 40 and 350 days old) were exposed to Co60 radiation for 44 days or for a total of 1000 r. Similar groups served as nonirradiated controls. When 201 and 451 days of age, respectively, all subjects were tested for discrimination learning. When 275 or 525 days of age, all subjects were tested for conditioned avoidance. The groups were differentiated on both tests, but with interactions involving age, irradiation, and testing conditions. The data indicate that behavioral change can be discerned following both natural- and radiation-induced aging. Further, the data indicate that the changes characterize, uniquely, these aging conditions.