The effect of the adenosine A2A receptor agonist CGS 21680 on GABA release from the striatum was investigated in vivo in young and old rats by microdialysis experiments. CGS 21680 at 10 microM significantly increased GABA spontaneous outflow in young but not in old rats. Conversely, CGS 21680 significantly increased potassium-evoked GABA release in old but not in young rats. The stimulating effect of CGS 21680 on spontaneous GABA release may be both secondary to an increased outflow of excitatory amino acids and to the removal, brought about by A2A receptor stimulation, of the inhibitory influence exerted by dopamine through D2 receptors on GABA-enkephalinergic neurones. The stimulating effect of CGS 21680 on potassium-evoked GABA release in old rats may be ascribed to an age-dependent imbalance in favour of adenosine vs. dopamine, recently described in the striatum of old rats.