Open-field activity levels of young (2- to 6-month-old) and older (12- to 30-month-old) C57BL/6J and DBA/2J mice were assessed in two experiments conducted in separate laboratories. A large decrement in activity level was observed with advancing age in mice of both genotypes. The environment in which the subjects were maintained and tested also appeared to be a major contributor to behavioral variability in the openfield.