In an earlier study Gruberg and Solish showed that salamander somatosensory spino-tectal fibers took the same course and ended in the same layer of the intermediate tectal neuropil as 5-hydroxytryptamine (5-HT or serotonin) fibers as seen by Falck-Hillarp fluorescence. In order to determine whether the spino-tectal fibers are themselves serotonergic, we have carried out anatomical, biochemical,...