An attempt has been made to determine the value of the lysine-8-vasopressin test when used together with the insulin-induced hypoglycaemia and the metyrapone test for the differentiation between hypothalamic and pituitary secondary adrenocortical insufficiency. This study was carried out in 65 children and adolescents with various disorders associated with growth retardation or overweight. The criteria for normal responses to the tests were based on findings in a 'control' group consisting of children with short stature without endocrinological disease. These criteria have been discussed in detail. In the control group, the concordance between the results of each of the tests was good. As far as the groups with proven or possible hypothalamic and/or pituitary disorders were concerned, discrepancies between the LVP test results and those of the ITT were found in a considerably higher proportion of cases. The combination of an abnormal ITT and a normal LVP test was encountered significantly more often than the reverse situation. This is in agreement with the hypothesis that LVP acts directly on the adenohypophysis while ITT acts at the hypothalamic or higher levels. The LVP test seems to be a valuable additionto the tests of hypothalamic-hypophyseal function.