A functional diagnosis of the diencephalohypophyseal system was carried out in patients with Sheehan syndrome, chromophobic adenoma, craniopharyngioma, prolactin-producing pituitary tumours, acromegaly, hypothalamo-pituitary dwarfism and constitutional retardation. A combined insulin hypoglycaemia/LH-RH/TSH test was performed to define frequency and extent of anterior pituitary insufficiency. With these illnesses, almost generally, a somatotropic insufficiency (except in acromegaly) was found. An impairment of gonadotropic function was often present, in general a pathologic LH-RH test correlating with a more or less developed androgen deficiency. An adrenocorticotropic insufficiency was found in most patients with sheehan syndrome, chromophobic adenoma and craniopharyngioma while in acromegaly and hypothalamo-pituitary dwarfism it was present less frequently, necessitating a substitution with corticoids. The TRH test reflects only incompletely a secondary hypothyroidism, and can be normal with organic processes of the diencephalo-hypophyseal region, making a T3 and T4 estimation in the blood decisive for a thyroid hormone substitution. A clear-cut separation of the hypothalamic from the pituitary cause of the insufficiency is neither possible with the LH-RH nor with the TRH test.