In this study, we investigated the distribution and developmental expression of the GABA_B receptor subunits, GABA_B1 and GABA_B2, in the main and accessory olfactory bulbs of the rat. Antibodies raised against these subunits strongly labelled the glomerular layer, suggesting that olfactory and vomeronasal nerve fibers express functional GABA_B receptors. Using postembedding immunogold cytochemistry, we found that GABA_B receptors can be present at both extrasynaptic and presynaptic sites of olfactory nerve terminals, and in the latter case they are preferentially associated with the peripheral part of the synaptic specialization. Olfactory nerve fibers expressed GABA_B1 and GABA_B2 at early developmental stages, suggesting that GABA_B receptors may play a role in olfactory development. Output and local neurons of the main and accessory olfactory bulbs were also labelled for GABA_B1 and GABA_B2, although the subcellular distribution patterns of the two subunits were not completely overlapping. These results indicate that presynaptically located GABA_B receptors modulate neurotransmitter release from olfactory and vomeronasal nerve fibers and that, in addition to this presynaptic role, GABA_B receptors may regulate neuronal excitability in infraglomerular circuits.