Exploratory, locomotor and ambulatory activities were measured in nine independent groups of three inbred strains of mice (BALB/cBy, C57BL/6J and DBA/2J), at three different ages (150 days, 400 days and 750 days). The results show that for two of the three variables, the age-related changes present different patterns as a function of strain. The genetic and/or environmental correlates of the reactivity to aging may thus depend on the behavioral trait under study.