In albino rats with bilaterally placed electrolytic lesions between the optic chiasma and paraventricular nuclei and in constant vaginal oestrus with no corpora lutea in the ovaries, luteinisation could be induced (1) by unilateral ovariectomy and hemiresection of the other; (2) by grafting half of one ovary after bilateral ovariectomy into the renal capsule or (3) into the spleen. While under the experimental conditions (1) and (2) the degree of luteinisation was found to be in a reciprocal relation to the amount of the remaining or grafted ovarian tissue, intrasplenic ovarian grafts (3) always contained a large number of corpora lutea. Formation of corpora lutea could accordingly be induced in rats with constant vaginal oestrus due to hypothalamic lesions by decreasing the oestrogen level in the blood. Luteinisation was most pronounced in the intrasplenic grafted animals in which a reduction of the oestrogen producing tissue had been combined with a practically quantitative oestrogen inactivation by the liver.