The morphological aspects of chromaffin tissue are reviewed, based mainly on our studies on the mouse adrenal gland. Particular attention was focused on the differential fixation of adrenaline and noradrenaline, and on the uptake and storage of [3H]dopa, [3H]dopamine and related substances in the adrenaline-storing (A) and noradrenaline-storing (NA) cells. Scanning electron microscopy combined with the NaOH-maceration method was useful for demonstrating the 3-dimensional organisation of nerve terminals, chromaffin cells, glial elements and vascular elements. In transmission electron microscopy, 3 types of chromaffin cell were distinguished. They were A, NA and SGC (small granule chromaffin) cells. After glutaraldehyde fixation followed by postosmication, A cell granules showed lower electron density, whereas NA cell granules were solid and dark. This difference in appearance between A and NA cells was first explained by the hypothesis that, after glutaraldehyde fixation, most of the adrenaline dissolved, whereas noradrenaline was precipitated in situ. Later, this hypothesis was supported by a series of autoradiographic and radioisotopic assay studies using [3H]dopa, [3H]dopamine and related substances; when [3H]adrenaline occurred, radioactivity in A cells mostly disappeared in the specimen, whereas that in the NA cells remained. At 15-60 min after an i.p. injection of [3H]dopa or [3H]dopamine, the concentration of radioactivity in A cells was higher than that in NA cells. However, in hypophysectomised mice, the radioactivity was low and evenly distributed in these 2 types of chromaffin cell. It was deduced that the carrier activities for extracellular dopa and dopamine were made greater in the A cells than in the NA cells by the pituitary gland. The hypophysectomy effects were restored by i.p. administration of ACTH. Images