We have compared the capacity to secrete ACTH in response to stress or adrenalectomy in control rats and in those with total hypophysectomy (H), adenohypophysectomy (AH) with preservation of the intermediate and the neural lobes, neurohypophysectomy (NH) with removal of the pars nervosa and all or part of the pars intermedia with preservation of the adenohypophysis, or incomplete adenohypophysectomy (IAH) in which a portion of the adenohypophysis and all of the pars intermedia and pars nervosa were left intact. Plasma ACTH measured with an N-terminal antibody that reacts on an equimolar basis with ACTH and alpha-MSH but not with other known pituitary hormones was elevated after ether or tourniquet stress in all except the H group. Three weeks after adrenalectomy there was an elevated basal plasma ACTH and an augmented ACTH response to stress in intact and IAH but not in AH rats. When a more specific alpha11-24 ACTH antibody was used there was a high plasma ACTH after ether stress in the IAH, NH, and intact groups but not in the AH or H groups. Adrenal weight and plasma corticosterone after tourniquet or ether stress were indistinguishable in the AH and H groups and were much higher and nearly identical in the intact, NH and IAH groups. We conclude that only the adenohypophysis secretes functionally significant amounts of ACTH. Plasma ACTH detected by the N-terminal antibody in the AH group is probably related to alpha-MSH or similar peptides and is incapable of maintaining adrenal weight or stimulating corticosterone secretion.