The effect of alpha-adrenergic blockade, by phentolamine, upon plasma levels of adrenocorticotrophic hormone (ACTH) and cortisol during insulin-induced hypoglycaemia was studied in pigs. Insulin administration produced a rise in ACTH which reached a maximum at about 30 minutes (p less than 0.01) and had returned to basal levels by 90 minutes. Cortisol levels had also risen by 30 minutes and remained elevated for 90 minutes. Pre-treatment with phentolamine almost completely suppressed the growth hormone response to insulin-induced hypoglycaemia, but had no effect upon the amplitude or duration of the elevation of plasma ACTH or cortisol levels following insulin administration. The hypoglycaemia-induced rise in ACTH and cortisol seem, therefore, to be independent of the alpha-adrenergic system. In addition, the inhibitory action of the alpha-adrenergic system on ACTH release reported in rats and dogs does not appear to be present in pigs.