An aldosterone-secreting adrenocortical adenoma was removed by surgery from a 15-year-old boy who exhibited signs characteristic of Conn’s syndrome including hypertension, potassium depletion and increased aldosterone production rate. The neoplasm was found to consist mainly of clear cells by light microscopy. Ultrastructural investigations revealed that the neoplasm was composed chiefly of fasciculata cells characterized by mitochondria with vesicular cristae. Some mitochondria, however, possessed tubular cristae reminiscent of those occurring in zona glomerulosa cells. It appears that mitochondria are not rigidly static constituents of cells but they can change constantly in order to satisfy variations in functional need. Other ultrastructural features included accumulation of lysosomes and lipofuscin granules, an increase of the rough-surfaced endoplasmic reticulum membranes, lipid depletion, focal hypertrophy and dilatation of the Golgi sacculi, and a pronounced hypertrophy of the smooth-surfaced endoplasmic reticulum. The latter alterations were thought to represent the fine structural manifestations of enhanced cellular activity associated with increased production of aldosteron