gamma-Aminobutyric acid (GABA) inhibits neurons by acting at GABAA and GABAB receptors but it is not known whether the two receptors are associated with discretely separate afferent inputs or whether GABA released from a single presynaptic neuron activates both receptors. Intracellular recordings were used to show that, in the lateral amygdala and ventral tegmental area of the rat, distinct sets of GABA-containing neurons provide the synaptic input to GABAA and GABAB receptors. Synaptic potentials resulting from GABAA receptor activation (blocked by bicuculline) and from GABAB receptor activation (blocked by 2-hydroxysaclofen) occurred spontaneously but as unrelated events. Furthermore, the two components of evoked synaptic potentials were differentially inhibited by agonists acting presynaptically (muscarine and 5-hydroxytryptamine). The finding that GABA acting at GABAA and GABAB receptors originates from distinct sets of presynaptic fibers suggests that two groups of GABA-containing neurons might be generally distinguishable in the mammalian nervous system.