The presence of somatostatin in afferent fibers impinging on the goldfish Mauthner (M-) cell was determined using immunohistochemical methods, combined with confocal and electron microscopy, and the relationship of this peptide with inhibitory and excitatory terminals was studied. Somatostatin-reactive boutons were present only on the distal part of the M-cell's lateral dendrite. Somatostatin immunoreactivity was observed in typical large myelinated club endings (LMCEs) corresponding to mixed (electrical and chemical) eighth nerve primary afferent fibers. The axoplasm of these fibers contained dense- core vesicles (DCVs) dispersed among round vesicles. We have made a novel finding that the excitatory transmitter glutamate is present in LMCEs. Colocalization of this amino acid with somatostatin was detected in 75% of these endings using postembedding staining with gold particles of various sizes. The other structures labeled by somatostatin antibody were found to be small vesicle boutons (SVBs), which establish symmetrical synapses and contain a population of pleiomorphic vesicles with DCVs scattered among them. Double labeling with antibodies against glutamic acid decarboxylase and GABA allowed the definition of three types of biochemically characterized terminals: [somatostatin-GABA], [GABA], and [somatostatin]. However, the occurrence of DCVs in SVBs stained for GABA alone suggests that neuropeptides other than somatostatin may also coexist with GABA in this class of boutons. The coexistence of somatostatin with both inhibitory and excitatory neurotransmitters acting on the same region of a postsynaptic cell is discussed in relation to the role postulated for this peptide in synaptic plasticity.