SUMMARY Although dehydroepiandrosterone sulphate has been recognized as one of the major secretory products of the human adrenal gland neither its biosynthetic pathway nor its site of formation within the cortex have been extensively studied. Ultramicrochemical techniques, which relate enzymic activity to well-defined groups of cells, have been used in the work now described to study the sulphation of dehydroepiandrosterone in the zones of the guinea-pig adrenal cortex. It has been shown that dehydroepiandrosterone sulphokinase activity resides only in the compact cell of the zona reticularis.