Objective. Because former investigations have reported abnormal changes in the expression of serotonin (5-hydroxytryptamine [5-HT]) and substance P (SP) in serum and cerebrospinal fluid, this study sought to determine whether 5-HT and pain-modulating neuropeptides (SP, galanin [GA], pituitary adenylyl cyclase-activating polypeptide, and secretoneurin) were expressed abnormally in the muscle tissue of patients with fibromyalgia (FM). Methods. Snap- frozen muscle tissue specimens (deltoid muscles) from 10 patients with FM (mean disease duration 15 years) and from 10 healthy control subjects were examined by reverse transcriptase-polymerase chain reaction (RT-PCR) of RNA preparations from muscle cells, and by immunohistochemistry methods (alkaline phosphatase-anti-alkaline phosphatase and immunogold-silver) using specific primers as well as antibodies. When specific messenger RNA (mRNA) was detected by RT-PCR, in situ RT-PCR was performed for mRNA localization. Results. Specific mRNA for the examined substances was absent in 9 of 10 FM patients and in 10 of 10 controls. No differences between the FM patients and controls could be detected in the muscle tissue by immunohistochemistry. In 1 FM patient, mRNA for the GA receptor could be shown. Conclusion. This study showed that 5-HT and neuropeptides are not produced in the muscle of patients with FM, and therefore do not appear to be involved in the peripheral induction of pain in this chronic, painful disorder.