5-Hydroxytryptamine (5-HT) localization has been studied in the spinal and trigeminal ganglia of adult rats using immunofluorescence, peroxidase-antiperoxidase immunocytochemistry and [3H] 5-HT uptake radioautography, the latter two at the ultrastructural level. Endoneurial mast cells, identified by alcian blue staining, formed 8 and 14% of all mast cells and neurons in spinal and trigeminal ganglia respectively and had a median diameter of 7.6 microns. Light and electron microscopic immunocytochemistry showed that these mast cells contained 5-HT-like immunoreactivity. Some 75% of them accumulated exogenous [3H]5-HT in vitro. A population of small-diameter neurons, which did not stain with alcian blue, was also labelled with anti-5-HT serum and accumulated [3H]5-HT. The possible roles of 5-HT in sensory ganglia are discussed.