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Plasma concentrations of prolactin, growth hormone, insulin and thyroxine have been examined in relation to mammary growth and lactation induced by oestradiol benzoate (OB) and progesterone in ten virgin female goats. A daily injection of OB (250 µg) and progesterone (60 mg) stimulated a significant, accumulative increase in prolactin in the circulation after 15 and 65 days and this was accompanied by udder growth. No increase in udder size was observed when prolactin was maintained at basal concentrations by simultaneous daily injections of the prolactin-inhibitor, bromocriptine (5 mg). Increasing the daily dose of OB (2·5 mg) and decreasing that of progesterone (6·25 mg) led to a further increase in both plasma prolactin and udder size and stimulated the onset of copious lactation. When prolactin was maintained close to basal concentrations throughout the periods of steroid treatment milk secretion was almost totally inhibited. There was some evidence that steroid treatment enhanced the prolactin response to milking and that additional quantities of bromocriptine were necessary to inhibit the secretion of prolactin when the pituitary gland was exposed to steroid stimulation. There were no marked changes in growth hormone, insulin or thyroxine in the circulation until the start of milking. It is concluded that the presence of increased quantities of prolactin in the blood are essential for the successful induction of mammary growth and lactation with OB and progesterone in virgin goats.

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