So-called small intensely fluorescent (SIF) cells were visualized immunohistochemically using a newly developed anti-dopamine (DA) serum in the rat superior cervical ganglia (SCG), both light and electron microscopically. Specificity of the anti-DA serum was tested in a control test and enzyme-linked immunosorbent assay (ELISA). DA-immunoreactivity was found in a subpopulation of SIF cells, but not in principal neurons. Ultrastructurally, DA-immunoreactivity was noticed in the cytoplasmic matrix and vesicles of perikarya, dendritic processes and terminals of SIF cells. DA-labeled terminals made symmetrical synaptic contacts with unlabeled principal neurons. These findings provide a morphological basis for the role of dopaminergic SIF cells as interneurons.