Physiological measurements suggest that the inhibition of primate spinothalamic tract cells by serotonin is mediated by myelinated axons. Previous morphologic studies emphasize that most serotonin-containing axons in the spinal cord are unmyelinated. Accordingly, the possibility that some serotonin-containing axons in the primate dorsolateral funiculus of the spinal cord are myelinated was investigated. Macaque monkeys were given L-tryptophan and the monoamine oxidase inhibitor, nialamide, intraperitoneally 1 h prior to sacrifice to increase axonal stores of serotonin. The animals were perfused (0.05 or 0.5% glutaraldehyde, 4% paraformaldehyde), and transverse sections of the thoracic cord were reacted with antibody against serotonin and then prepared for electron microscopy. Many of the immunostained axons in the dorsolateral funiculus included fine, myelinated fibers with diameters of 0.7-2.2 microns. Unmyelinated serotonin-containing axons were also observed. The observation of myelinated serotonin-containing axons in the white matter of the monkey dorsolateral funiculus contradicts the view that the descending serotoninergic projection consists entirely of unmyelinated fibers, particularly since the conduction velocity of the fine fibers would be too slow to account for the earliest latency of descending inhibition following stimulation in the brainstem. The presence of myelinated serotoninergic axons presumably accounts for the latencies reported for the inhibition of primate spinothalamic cells following stimulation of the periaqueductal gray, an inhibition that can be blocked with serotonin antagonists and that is associated with the release of serotonin in the dorsal horn.