Ovariectomy on day 19 of pregnancy augmented galactosyl transferase activity 24 h after surgery preceding by 6 h the significant alpha-lactalbumin accumulation. Progesterone, injected immediately after ovariectomy showed a clear inhibitory effect on both galactosyl transferase and alpha-lactalbumin concentration, measured 30 h after ovariectomy. However, once the synthesis of lactose has been induced, progesterone is no longer inhibitory. Oestrogen induced a significant increase in lactose synthetase activity but no effect was obtained on galactosyl transferase activity. Progesterone, in a time and dose dependent relationship, was capable of preventing the effect of estrogen on lactogenesis. The lactogenic action of oestrogen in ovariectomized pregnant rats might be due to a direct effect at the mammary gland level facilitating the action of prolactin or through an indirect effect mediated via an increase on prolactin release.