Whole-cell recordings were made from substantia nigra pars reticulata in rat midbrain slices to study the functional expression of pre- and postsynaptic GABA B receptors in GABA output neurones. Baclofen (up to 300 μM) dose-dependently activated a weak current which was insensitive to tetrodotoxin and Ca 2+-free solution but blocked by Ba 2+ and 2-OH-saclofen. The maximum current activated by baclofen (30 μM) was 43.0±4.5 pA ( n=27), representing only 23% of that in dopamine neurones. Baclofen (1–30 μM) also reduced the frequency of the GABA A receptor-mediated miniature inhibitory postsynaptic currents while the distribution of their amplitudes was unaffected. This presynaptic effect of baclofen, prominent at a concentration as low as 1 μM, was sensitive to 2-OH-saclofen and occluded by Cd 2+, but was unaffected by Ba 2+. The results suggest a predominant role of the presynaptic GABA B receptors in substantia nigra pars reticulata. The relative abundance of pre- and postsynaptic GABA B receptor subtypes in this brain region may also be important in mediating the anticonvulsant effect of baclofen in rats.