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Morphological differentiation was enhanced in mammary glands from mice injected with oestradiol as neonates both in vivo, after 9 days of pretreatment with oestrogen and progesterone, and in vitro, either after 6 or 9 days of pretreatment and on all hormone supplements, or after culture with oestradiol, progesterone, aldosterone, insulin and thyroxine with or without prolactin and growth hormone. These hormone combinations further enhanced development, whereas culture with insulin and thyroxine did not.

Differentiation in vitro was least on medium containing insulin + thyroxine, and was greatest on the complete medium (insulin, thyroxine, growth hormone, prolactin, aldosterone, progesterone and oestradiol). An intermediate degree of differentiation occurred when growth hormone and prolactin were omitted from the complete medium.

Differences in lobule formation between oestrogen-treated and control mice were probably ovary-dependent at the dose used (25 µg/day), since pretreatment with oestradiol and progesterone was necessary to reveal them.

Enhanced hormone sensitivity of murine mammary tissues exposed to oestradiol early in life may be related to subsequent enhanced dysplastic potential of the tissue and provides a further basis for caution in the administration of steroids to immature individuals.