δ-Guanidinovaleric acid (GVA) is an endogenous convulsant and is thought to be a specific GABA antagonist. In the present study, we examined the effects of GVA and GABA agonists, GABA, muscimol and baclofen, on dopamine and serotonin releases in rat striatum using a brain dialysis technique. GVA produced a significant increase in DA released transiently (1mM) and throughout the experiment (10mM) compared with the controls. It also produced a significant increase in 5-HT release in both concentrations throughout the experiment. GABA (10mM) inhibited DA and 5-HT releases induced by GVA. Muscimol (10mM) inhibited DA and 5-HT releases induced by GVA. Especially muscimol was more effective in the inhibition of 5-HT release. Baclofen (10mM) inhibited only DA release induced by GVA. These results suggest that the activation of GABA receptor inhibits the release of DA and 5-HT in the striatum, and that the DAergic system regulates the GABA-B receptor while the 5-HTergic system mainly regulates the GABA-A receptor.