The paraganglia of adult man were studied using the formaldehyde-induced fluorescence (FIF) method for histochemical characterization of biogenic monoamines. Microspectrofluorimetry was used to record the emission spectra and fluorescence intensities of the paraganglionic cells. The study of samples from six patients showed that well vascularized paraganglia were widely distributed throughout the retroperitoneal spaces. The paraganglia exhibited strong FIF with the spectral characteristics of monamines. Treatment with HC1 caused an increase in the fluorescence intensity of the paraganglia and a simultaneous shift of the emission maximum from 480--495 nm. This change suggests the presence of high concentrations of tryptophyl-containing peptides and is not due to monoamines. The possibility of a dual endocrine function for the paraganglia is discussed.