Following hypophysectomy in rats, corticosterone secretion in vivo declined to a minimum within 24 hr. Cholesterol sidechain cleavage (SCC) activity per unit adrenal weight was also maximally reduced in that interval. Further losses in total SCC activity with time resulted from losses in adrenal weight rather than changes in unit enzyme activity. Administration of ACTH to rats hypophysectomized for 1 week restored unit SCC activity to control values within 48 hr. Corticosterone secretion also increased but remained far below control levels at that time. More prolonged treatment continued to increase corticosterone secretion without further affecting unit SCC activity. The results indicate that changes in SCC activity generally parallel changes in corticosterone secretion in vivo, but that diminished adrenal “responsiveness”to ACTH one week after hypophysectomy cannot be attributed to diminished SCC activity. (Endocrinology94: 1346, 1974)