Experiments to determine whether in vitro secretion rates of adrenocortical glands reflect pre-existing in vivo secretory potentials were conducted using California ground squirrels (Spermophilus beecheyi). When the excised glands from ACTH-injected squirrels and from controls (untreated squirrels shot in the field) were incubated in ACTH-supplemented media, Cortisol production was significantly greater in the ACTH-injected squirrels.

The rate of corticosterone production in glands from ACTH-injected squirrels— in contrast to the Cortisol relationships — was less than or equal to that from controls. The difference was significant in glands incubated without ACTH but not in those incubated with ACTH.

Thus in vitro Cortisol production reflects in vivo production. The same may also be true for corticosterone if it can be shown that ACTH injection decreases corticosterone production.