Summary. To evaluate the relative role of "diabetogenic" hormones as insulin antagonists in severe derangements of diabetic control, glucagon, cortisol, growth hormone and adrenaline were administered by continuous intravenous infusion, separately and in combination, to ketosis-prone insulin-dependent diabetics (n = 11). The amount of insulin required for the assimilation of a 50 g glucose load during the various hormone infusions was determined by means of an automated glucose-controlled insulin infusion system and used as an index of insulin effectiveness. Raising plasma hormone concentrations acutely into the range seen in severe diabetic states (glucagon 517+_70pg/ml; cortisol 32-+3gg/dl; growth hormone 14+3ng/ml) did not alter significantly blood glucose profile and insulin requirement (control ll.3_l.lU; glucagon 11.6-+2.0U; cortisol 11.1_+0.4 U; growth hormone 12.9___1.4 U), except for adrenaline (plasma level 550_+ 192 pg/ml), which caused a marked rise in blood glucose levels and a threefold increase in insulin demand (31.1_+ 3.7 U). Combined infusion of all hormones did not potentiate significantly the latter effect (38.3___4.7 U). The effectiveness of metabolic control by insulin was assessed by a marked decrease in plasma nonesterified free fatty acids and ketone bodies upon its administration after glucose ingestion in all groups studied. It is concluded that from the hormones investigated within this study adrenaline exerts the strongest diabetogenic action during its short term administration followed by that of growth hormone. Whereas it may well be that over-insulinization of the patients by the glucose controlled insulin infusion system has overcome and disguised the smaller diabetogenic effects of cortisol and glucagon.Key words: Diabetes mellitus, counter-regulatory hormones, glucagon, cortisol, growth hormone, adrenaline, insulin, non-esterified fatty acids, ketone bodies.Excess secretion of insulin-counteracting hormones is claimed to be important in the metabolic changes and insulin resistance in diabetic ketoacidosis [1,2]. The observations in severe derangements of diabetic control include elevated plasma levels of glucagon [3], cortisol [4], growth hormone [5,6], and catecholamines [7,8]. The hierachy of their pathogenetic significance, however, remains to be established. Hyperglycaemia, decreased glucose tolerance and insulin resistance have been demonstrated under certain circumstances after exogenous administration of these hormones [9][10][11][12]. Furthermore, the above mentioned counter-regulatory hormones are known to exert lipolytic and ketogenic activity when elevated to high physiological concentrations in insulin deficient diabetic man [12][13][14][15][16]. The delay in the development of ketoacidosis in insulin-withdrawn diabetics by either somatostatin-induced suppression of growth hormone and glucagon [17] or following pituitary ablation [1] serves as further support for implicating these hormones as pathogenic factors of deranged metabolic control. In addition, circulator...