Employing the standard vaginal smear technique as the basis of study, it has been noted that cortisone treatment prevents the inhibition of oestrous cycle in alloxan diabetic rats, which is explained as possibly due to the prevention of a competitive inhibition of the adenohypophyseal gonadotrophin release by the excessive production of ACTH or adrenal cortical growth factor. © 1966 S. Karger AG, Basel.