The present study showed the acetylcholinesterase (AChE) activity, and 
neurofilament protein (NFP), catecholamine-synthesizing enzymes, dopamine 
beta-hydroxylase (DBH) and phenylethanolamine N-methyltransferase (PNMT) 
immunoreactivities in the mouse adrenal gland during postnatal development. From 
birth to postnatal-1-day, AChE activity was weakly and diffusely found in some 
medullary cells and in very few nerve fibers whereas strong NFP immunoreactivity 
was seen in a few ganglion cells and in remarkably numerous nerve fibers in the 
medulla. Almost all meduallary cells were reactive for both DBH and PNMT during 
this period. From postnatal-2- or -3-day to postnatal-1-week, strong AChE 
activity was observed in a few large ganglion cells, but the reaction was weak 
in clusters of chromaffin cells, and the number of strong AChE-active nerve 
fibers in the medulla was rapidly increased. From postnatal-2-day onwards, the 
number of NFP-immunoreactive nerve fibers in the medulla were remarkably 
numerous. Numerous chromaffin cells were reactive for both DBH and PNMT whereas 
some chromaffin cells were reactive for only DBH from postnatal-2-day onwards. 
These results suggest that drastic changes such as an increase of acetylcholine 
in the nerve fibers, differentiation of noradrenaline and adrenaline cells of 
the medulla may occur during this period. From postnatal-2-week to 
postnatal-3-week, weak AChE activity was seen in the clusters of several 
chromaffin cells and a few ganglion cells, and the number of AChE-active nerve 
fibers in the medulla was gradually increased. From postnatal-4-week to 
postnatal-8-week (adult), the distribution and frequency of AChE activity in the 
adrenal gland were similar to those at postnatal-3-week. In the adult, AChE 
activity was weakly seen in the clusters of several chromaffin cells showing 
noradrenaline fluorescence in the adrenal medulla. The noradrenaline cells were 
contacted by denser AChE-reactive nerve fibers than adrenaline cells. These 
results suggest that the development of cholinergic nervous system in the mouse 
adrenal medulla may be completed by postnatal-3-week.