SUMMARY The coated charcoal immunoassay method was applied to the measurement of the corticotrophin (ACTH) response to insulin hypoglycaemia, lysine-vasopressin, metyrapone and surgical stress in female piglets. The maximum ACTH responses average 392 picograms (pg.)/ml. (insulin hypoglycaemia), 111 pg./ml. (lysine-vasopressin) and exceeded 350 pg./ml. after metyrapone. The time relationships between blood sugar, 11-hydroxycorticosteroid and ACTH levels were also examined. Despite constant infusion rates, the ACTH response to lysine-vasopressin was not sustained. Previous administration of dexamethasone suppressed the response to insulin hypoglycaemia and lysine-vasopressin.