The question of whether the age-related decline in physical activity reported in humans has a biological basis is addressed by reviewing gerontological studies that have used nonhuman subjects. From at least three separate arguments, this review provides strong support for a biological basis of this phenomenon. First, age-related decline in activity measured in many different ways is observed across a wide range of nonhuman species. Second, the activity decline appears predictive of lifespan. Increased levels of activity predict longevity, and increasing activity through exercise increases median lifespan. Third, activity declines appear related to altered neurotransmission involving the central dopamine system. Reduced dopamine release or loss of dopamine receptors appears to underlie age-related activity decline, and interventions that enhance dopamine function can increase activity levels in aged animals.