Daily administration of 1 μg EB plus 3 mg P to mature ovary-thyro-para-thyroidectomized rats for 19 days stimulated mammary gland growth (DNA) comparable to ovariectomized rats receiving same treatment. Replacement therapy with 10 and 20 μ AT 10/100 g/day, respectively, in combination with 1 μg EB plus 3 mg P increased total DNA significantly. No benefit was observed by increasing ovarian hormones to 2 μg EB plus 6 mg P. Combined administration of AT 10 at above mentioned levels and 1.5 μg and 3 μg T4/100 g depressed DFFT significantly resulting in reduced total DNA.