The age-associated decline of dopamine D2 receptor (D2R) densities is one of the notable features of aging in mammalian brains. The concept of a functional dopamine (DA)-acetylcholine (ACh) interaction in the striatum has been strongly supported. However, the effects of this decline of D2R on the striatal DA-ACh interaction still remain to be elucidated. In the current study we examined the effects of different concentrations of a D2R agonist, quinpirole, on the striatal ACh release in three groups of rats at different ages (6-, 15-, 25-months-old) by in vivo microdialysis. The ACh release in the striatum was significantly decreased by quinpirole infusion in all three age groups. The old rats showed a significantly smaller decrease of the ACh release by quinpirole than that of the middle-aged rats at the dose of 0.1 microM and that of the young rats at the dose of 1 microM. The current study suggested that the striatal DA-ACh interaction was affected by the aging process.