The GABAB agonist baclofen has been shown to suppress synaptic transmission in subregions of the hippocampus and in the piriform (olfactory) cortex. Here we report a laminar selectivity of suppression of synaptic potentials in the olfactory cortex. In brain slice preparations, baclofen suppresses extracellularly recorded field potentials at the intrinsic fiber synapses proximal to the superficial pyramidal cell bodies (layer Ib) while leaving the afferent fiber synaptic potentials recorded at the distal dendrites (layer Ia) little affected. This dose-dependent selective suppression of intrinsic fiber synaptic transmission is also correlated with an increase of paired-pulse facilitation. These results suggest that afferent and intrinsic synaptic inputs may be differentially modulated by the activation of GABAB receptors and that this selective suppression is at least partially mediated via a presynaptic mechanism.