The nerve plexuses and ganglionic nerve cells of rat trachea were studied quantitatively in whole-mount preparations stained for acetylcholinesterase. Two nerve plexuses were observed overlaying the smooth muscle layer of the membranous part or the rat trachea. On average, 70 ganglia (range 63-78, S.D.=6) and 549 neurons (range 334-661, S.D.=125) were found on the nerve plexuses. About 80% of ganglia are distributed in the thoracic part of the trachea. The ganglia are relatively small, 77% of the tracheal ganglia contain only 5 (or fewer) neurons. The profile area of neurons are about 474 芦m2 (range 207 "m2-918 "m2). It appears that the uneven distribution of neurons is attributed to the increased volume of the thoracic tracheal smooth muscle. The ultrastructural features of the rat tracheal ganglia is similar to that of other species. A few regular processes of fibroblasts form the perineurium of the ganglia. Collagen fibrils, myelinated and unmyelinated nerve fibers and blood vessels are present inside the ganglia. The presence of muscular blood vessel within the tracheal ganglion has nerve been reported before. This may further support the possibility that the tracheal ganglion receives a separate blood supply.