Simultaneous determinations of plasma ACTH, HGH and Cortisol were carried out in the course of an insulin tolerance test (ITT) in 7 normal subjects and in different cases of hypothalamic pathology. The parallelism observed in normal subjects between the different hormones disappeared in cases of clinical hypopituitarism with hypogonadotrophic hypogonadism: the HGH response was almost absent, whereas the ACTH response to ITT was above normal. Complete disappearance of the HGH response with very low ACTH levels was observed in 3 cases of chromophobe adenoma and in one case of hypothalamic glioma with diabetes insipidus. The ACTH radioimmunoassay appears to be useful in the study of the pathology of the hypothalamus-pituitary-adrenal axis, mostly during dynamic tests with simultaneous measurement of other pituitary hormones. The ACTH response is more rapid, more sensitive and more reliable than the indirect Cortisol response. 1 Presented in part at the round table conference "ACTH in blood", 18th Symp. Dtsch. Ges. Endokrinol., Hanover (Germany), March 1-4, 1972