5, 6-dihydroxytryptamine-induced pigmentation of serotonergic neurons was combined with retrograde cobalt-or nickel-lysine labelling in order to identify serotonergic cell bodies among cobalt-or nickel-labelled ones. Cobalt-or nickel-labelled serotonergic cell bodies could easily be distinguished from labelled, nonserotonergic neurons. The nonserotonergic cobalt-labelled neurons have only orange, whereas the nickel-labelled neurons have only blue colorization in their somata, while the labelled serotonergic neurons have dark pigment granules in their somata besides the blue or orange colorization. By this double labelling technique the central serotonergic neurons which send processes to different body regions through CNS nerves, can be identified in wholemount preparations.