Progesterone alone is responsible for stimulation of the growth of ducts and of mammary alveolar structures in mice.

The effect of estradiol valerate and progesterone on mammary growth of prepubertal (20, 25 and 30 days of age) female and prepubertal (15, 20 and 30 days of age) and adult male C3H/Di mice were studied. Estradiol treatment had no effect on mammary growth in 30 day old females, but mammary growth of 20 and 25 day old females showed clear inhibition. In contrast, treatment with progesterone alone stimulated both the growth of ducts and formation of alveolar buds. In males, the injection of estradiol valerate resulted in a stimulation of the growth of mammary rudiment. The treatment with progesterone alone stimulated the growth of mammary ducts and the formation of alveolar buds in males treated from day 15 and 20 of age, while mammary growth of those treated from day 30 of age was low and the growth of mammary rudiment of adult males was not stimulated at all. The mammogenic effect of progesterone was not affected by time of weaning of males but it was inhibited by simultaneous application of testosterone. An age dependent decrease of the mammogenic effect of progesterone in males is related to an increase in plasma concentration of testosterone.