Nitric oxide (NO) is a free radical postulated to act as a neurotransmitter, neuromodulator, or second messenger molecule in the central nervous system. Several findings suggest that NO production may be decreased in the aged rats. In the present study, we investigated regional discrepancies in changes with aging in the number of nitric oxide synthase-immunoreactive (NOS-IR) cells in the basal ganglia of the aged rat by immunocytochemistry. The number of NOS-IR neurons in the striatum and substantia innominata of the aged rat decreased. In contrast, the number of NOS-IR neurons in the subthalamic nucleus increased in the aged rat. On the other hand, the number of NOS-IR neurons in the nucleus accumbens and olfactory tubercle did not change. Taken together, important functional changes can be caused by the region-specific changes of NOS-IR neurons in the basal ganglia with aging.