It has been proposed that the FOS protein encoded by c-fos protooncogene functions as a nuclear "third messenger" molecule that couples short-term extracellular signals to long-term alterations in cell function, by regulating the expression of specific target genes. In the present study, immunocytochemical double staining technique was used to investigate the effects of electroacupuncture on the expression of c-fos oncogene in the serotonergic neurons in the nucleus raphe dorsalis (NRD) that has been known to play an important role in the endogenous analgesic system of the brain. The number of FOS positive serotonergic cells in the NRD increased significantly after the electroacupuncture stimulation. These results indicate that electroacupuncture can activate central serotonergic neurons at gene expression level.