It is a well-known fact that hypophysectomy leads to a decrease in the size and weight of the adrenal glands with diminution of the lipoid content of the cortex and consequent development of a sudanophobe zone in the zona fasciculata. Administration of anterior pituitary extracts in hypophysectomized animals causes an increase in the size and weight of the adrenals, restoration of the lipoid content and disappearance of the sudanophobe zone. Various methods have been proposed for the standardization of such pituitary corticotrophic extracts. Collip, Anderson & Thompson [1933] removed and weighed one adrenal from rats 10 days after hypophysectomy; the rats were then treated with extract and the increase in weight of the remaining adrenal was measured. They had already established that after removal of the first adrenal under these conditions there is no compensatory hypertrophy of the second, owing to the absence of the pituitary corticotrophic action. They found