The knowledge that the ovaries are the organs mainly responsible, or as is generally believed, entirely responsible for the control of the cyclical changes which occur in the accessory reproductive organs is based upon three sets of observations: (i) those which show that ovariectomy is followed by the apparent cessation of all cyclical activity in the accessory organs; (ii) those which show that the phases of the ovarian cycle are correlated with the cycle of changes that take place in the accessory reproductive organs; and (iii) those which show that the latter changes can be reproduced in spayed animals by the administration of ovarian hormones. The view that the adrenal cortex is also concerned in the control of the cyclical changes to which the accessory reproductive organs are subject is so far based entirely on the fact that threshold artificial oestrous cycles [Bourne & Zuckerman, 1941] cannot, as a rule,