By the use of immunocytochemical staining methods, we studied the morphology and distribution of 5HT and NPY immunoreactive cells and fibres in the mouse adrenal gland. The 5HT-immunoreactive cells were numerous and widely localized in the medullar tissue. These cells were arranged in three cellular types with regard to their morphological and immunocytochemical features. One of them showed cells with polygonal shape, being intensified like the typical medullary chromaffin cells. These immunoreactive cells were observed arranged in medullar islets. The second 5HT-immunoreactive cellular type was constituted by cells with polygonal shape and strong immunoreactivity. The third one was formed by cells with immunoreactive prolongations. We found some islets of chromaffin non-immunoreactive cells surrounded by immunostained cells. We also observed some 5HT-immunoreactive nerve fibres in the medullar tissue. NPY-like immunoreactivity was detected in both chromaffin and ganglion cells in adrenal medulla. NPY-like immunoreactivity was also detected in nerve fibres at cortical level. In a few cases, we observed medullar 5HT- and NPY-immunoreactive tissue in the adrenal cortex (monotremas).