We attempted to determine the existence of a functional serotonergic small intensely fluorescent (SIF) cell in the rat superior cervical sympathetic ganglion (SCG) by evaluating the effects of pargyline, decentralization and electrical stimulation on the preganglionic sympathetic fibers. The objective was to assess changes in serotonin (5-HT) metabolism compared with findings in the case of dopamine (DA) metabolism. The contents (ng/ganglion, n = 8, average +/- S.E.) in the adult male Wistar rat SCG of 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) were 1.19 +/- 0.11 and 0.11 +/- 0.01, respectively, determined by our reverse-phase high-performance liquid chromatographic electrochemical method. A relatively large amount of 5-HT was detected in the capsule of the SCG (0.62 +/- 0.16 ng/capsule, n = 8) without detection of 5-HIAA. The pargyline injection increased the DA content and decreased the 3,4-dihydroxyphenylacetic acid (DOPAC) content rapidly. The increase of the 5-HT content was observed without changes in the capsule 5-HT. The mode of pargyline induced-increase in the 5-HT content and decrease of the 5-HIAA content was slow compared to that of DA and DOPAC. Decentralization did not alter the 5-HIAA content, whereas the DOPAC content was markedly reduced. Although electrical stimulation significantly increased the DOPAC content, changes in the 5-HIAA content were not observed. It would thus appear that 5-HT is produced in the rat SCG, and if so, then support for existence of a serotonergic SIF cell in the rat sympathetic ganglion can be obtained. A direct connection between pre- and postganglionic sympathetic neurons through the serotonergic SIF cell was not demonstrated.