Serotonin neurons in nucleus raphe dorsalis and paragigantocellularis of the cat contain enkephalin.

1. The nucleus raphe dorsalis (RD) and paragiganto-cellularis (PGL) of the cat contain both 5-HT and leucine-enkephalin (ENK) immunoreactive perikarya. 2. Using a sequential immunoperoxidase-immunofluorescence technique for the localization of ENK and 5-HT, respectively, it was demonstrated that some PGL and RD neurons contain both the indoleamine and the peptide. 3. The double labeled neurons in the DR were characteristically small, round cells predominantly located on the midline of the nucleus, dorsal to the medial longitudinal fasciculus. Numerous large 5-HT containing perikarya were also found in the DR, but these cells did not contain ENK immunoreactivity.