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 × increase in number of PG cells between 3- and 33-month old rats. Microspectrofluorimetric quantitation in old rats showed equal amounts of catecholemines in PG cells and in adrenal medullary cells. Most PG were located in samples from the para-aortic area.