1. The effect of baclofen, a GABAB receptor agonist, on the release of dopamine from the striatum (ST), substantia nigra (SN) and prefrontal cortex (PFC) of the rat was examined by intracerebral microdialysis. 2. Perfusion of baclofen 50 microM did not affect the striatal release of dopamine. However, dopamine release was markedly reduced in the SN and PFC. 3. 3,4-Dihydroxyphenylacetic acid and homovanillic acid output increased in the ST and decreased in the SN and PFC when baclofen was perfused through the microdialysis probe. 5-Hydroxyindoleacetic acid levels were not affected in any experimental condition by baclofen perfusion. 4. The results suggest that GABAB receptors modulate the release of dopamine in the SN and PFC, but do not affect the striatal release of dopamine, which indicates that the role of GABA receptor activation is different in the dopaminergic terminals of the ST and PFC.