Glial cell line-derived neurotrophic factor has been shown to affect dopaminergic and cholinergic neuron markers and functions in young rats. However, it is not known if the response to exogenous glial cell line-derived neurotrophic factor is augmented during normal aging. Thus, the effects of chronic intraventricular infusions of glial cell line-derived neurotrophic factor were determined in young adult (three-months-old) and aged (24-months-old) Fischer 344 (F344) male rats. The effects of glial cell line-derived neurotrophic factor were compared to the effects of the neurotrophin nerve growth factor. Growth factors were administered at a dose of 10 mg/day for 14 days. Locomotor activity and weight changes were also examined in all rats. Aged F344 rats showed significantly reduced (by 75-80%) locomotor activity compared to young rats. In glial cell line-derived neurotrophic factor-treated aged and young rats there was significantly increased (242% and 149%, respectively) locomotor activity measured at seven days. There was also a significant increase in locomotor activity measured 14 days after the start of infusion. Both glial cell line-derived neurotrophic factor and nerve growth factor reduced weight gain by 10% in young and old F344 rats. Two weeks following the start of nerve growth factor or glial cell line-derived neurotrophic factor administration the brains were used for neurochemical analyses. Glial cell line-derived neurotrophic factor significantly increased tyrosine hydroxylase activity in the substantia nigra and striatum of aged rats and in the substantia nigra of young rats. Nerve growth factor treatment did not significantly affect tyrosine hydroxylase activity. However, glial cell line-derived neurotrophic factor and nerve growth factor increased choline acetyltransferase activity in the septum, hippocampus, striatum and cortex of aged rats and in the hippocampus and striatum of young rats to a comparable degree. These findings indicate that specific dopaminergic and cholinergic neuron populations remain responsive to glial cell line-derived neurotrophic factor during the life span of the rat and may be involved in maintaining phenotypic expression within multiple neuronal populations. Additionally, the glial cell line-derived neurotrophic factor-induced up-regulation of brain neurotransmitter systems may be responsible for increased locomotor activity in F344 rats.