For identification of paraganglia (PG), samples of para-aortic tissue, tissues containing the coeliac-mesenteric ganglion complex, and the hypogastric ganglia were removed from 3- and 33-month-old male Fischer-344 rats and were processed by the formaldehyde-induced fluorescence method for visualization of catecholamines. Small PG containing 5-30 cells per section were found consistently in young animals. In each of six old rats, large PG containing 500-4000 brightly fluorescent cells per section were detected. Cell counting revealed a 13.5 x increase in number of PG cells between 3- and 33-month-old rats. Microspectrofluorimetric quantitation in old rats showed equal amounts of catecholamines in PG cells and in adrenal medullary cells. Most PG were located in samples from the para-aortic area.