The weight and shape of the adrenal medulla were studied in 118 adrenal glands obtained at autopsies of 62 patients (38 males and 24 females) between 0 and 52 years of age. In adolescents 15 years and older and adults cases of sudden death only were entered in this study. The weight was calculated using morphometric measurements done on serial sections of the glands. In the new-born, the medulla accounts for less than 1% of the total volume of the adrenal gland. Though there is a rapid growth of the adrenal medulla after birth, the percentage of adrenal medullary volume at all age levels during childhood and adolescence is lower than in adults where it constitutes 9% of the total adrenal volume on the average. This corresponds to an average medullary weight of 0.43 g. In the new-born, the medulla consists of a thin plate made up of immature medulloblasts. Within a few months these are transformed into mature medullary cells; the shape of the medulla soon approaches that of the adult gland: an increase in thickness around the central vein and flat processes into the alae. With increasing age the cortico-medullary border becomes irregular and ragged. Especially in the vicinity of the central vein an intermingling of medullary and cortical cell complexes is found. An unequivocal diagnosis of genuine adrenal medullary hyperplasia can be established only by weighing the dissected medulla or by applying morphometric methods.