Two general categories of GABA-immunoreactive (GABA-Ir) boutons are present in the monkey basilar pontine nuclei (BPN). One type characterized by a pale or lucent appearance is involved both in glomerular arrangements that include serial, triadic synapses, and in non-glomerular synapses. The second type of GABA-Ir bouton exhibits a wide variety in size and shape and contains a greater complement of synaptic vesicles than the first type, giving it a darker appearance in comparison to the pale GABA-Ir boutons. Such boutons participate only in non-glomerular synapses. It is suggested that the pale GABA-Ir boutons arise from the intrinsic population of GABA neurons, while the darker appearing boutons might take origin from one of the GABA-ergic afferent systems that reach the BPN.