Drebrin (developmentally regulated brain protein)-like immunoreactivity was investigated in the adult rat mesencephalic trigeminal nucleus (MTN) using light and electron microscope. Intense immunoreactive puncta were observed on the cytoplasmic membrane and within the cytoplasm. The cytoplasm was also faintly immunopositive for drebrin, and thus MTN somata other than multipolar cells were distinguishable from non-MTN somata. These immunoreactive cell bodies were localized from the level of the superior colliculus to the pons. Electron microscopic observation showed that the post-synaptic cytoplasmic membrane at axo-somatic synapses was immunoreactive for drebrin. Drebrin-like immunoreactivity was also observed on spine-like processes emanating from MTN somata. In addition, the post-synaptic cytoplasmic membrane at axo-somatic synapses was also immunopositive for drebrin. Within the cytoplasm of MTN cell bodies, a part of the rough endoplasmic reticulum and neighboring structures were also immunopositive. Further, both ends of the somato-somatic close appositions that contained neuronal gap junctions harbored immunoreactive structures. We can infer from the results that drebrin is an ideal marker protein for MTN cell bodies. The abundance of drebrin-like immunoreactivity in the MTN neurons suggests that the MTN has highly flexible synaptogenesis.