The crest synapse and its related structures in the locus coeruleus (LC) of the Japanese monkey, Macaca fuscata, were electronmicroscopically studied using noradrenaline (NA) immunohistochemistry and tannic acid fixation. About 20% to 25% of the noradrenergic input to the LC terminated at noradrenergic neurons. About 1/3 of these NA-NA connections formed crest synapses, although conversely, NA-immunoreactive (IR) terminals were found in 10% of all terminals forming crest synapses. The crests showing parent dendrites were clearly NA-IR, and definite nonNA-IR dendritic crests were not found. Thus, crest synapses were present almost exclusively on noradrenergic neurons in the monkey LC. The crests were composed of parallel postsynaptic membranes 120 to 130 nm in width and 550 to 610 nm in length, accompanied by a thick-dense material of cytoplasm, and regular single rows of electron dense subjunctional bodies which were arranged in approximately 8 transverse rows. A maximum of 12 electron-dense subjunctional bodies per row were counted in the middle section, and decreased bilaterally in number. The crests further contacted with the dendritic spiny processes of neighboring NA-IR neurons. These results suggest that the crest synapse is an evolved and powerful synaptic structure which makes it possible for noradrenergic neurons in the LC to fire simultaneously at a slow and constant frequency depending on the behavioral state.