When placed in a tank of water, aged rats (24 to 27 months old) showed marked impairments in swimming. Compared with young adult rats (3 to 4 months old), the older animals moved their limbs less vigorously and were less successful in keeping their heads above water. The young, but not old, rats maintained a position nearly horizontal to the water surface and planed across it. These movement dysfunctions of aged rats resemble those seen in young adult animals that have sustained injury to brain dopamine-containing neurons. The swimming impairments of the aged rats were reversed by the dopamine receptor stimulant apomorphine and by the biosynthetic precursor of dopamine, L-dopa. Thus, age-related alterations in brain dopaminergic systems may be responsible for some of the movement disturbances associated with senescence.