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PRL involvement in periparturient mammary development and onset of milk secretion was studied in 17 multiparous, monotocous Holstein cows. Mammary glands were obtained 10 days prepartum from untreated cows and 10 days postpartum from untreated cows and cows treated with CB154 (2-Br-a-ergokryptin) or CB154 plus PRL. CB154 was administered for 12 days before expected parturition through 10 days postpartum. PRL was infused continuously for 6 days immediately before parturition in cows receiving CB154 plus PRL. Treatment with CB154 reduced basal serum PRL concentrations approximately 80% and blocked the normal surges in serum PRL concentrations at parturition and during milking. Average milk production in cows given CB154 alone was 11.4 kg/day lower than in controls. Periparturient infusion of PRL in cows treated with CB154 (CB154 plus PRL) prevented reductions in milk production. Treatment with CB154 had no effect on feed intake or periparturient serum concentrations of GH, progesterone, and glucocorticoids. Mammary glands from postpartum controls or cows treated with either CB154 or CB154 plus PRL contained equivalent total amounts of DNA. Inhibition of PRL secretion reduced total mammary RNA content 36% and decreased the RNA/DNA ratio (1.38) relative to ratios in cows that received PRL replacement therapy (2.17) or untreated lactating controls (1.86). Thus, reduced milk yields among cows treated with CB154 were associated with reduced mammary secretory cell activity rather than reduced cell numbers. Biosynthetic rates of fatty acid and lactose synthesis were lower in mammary tissue from cows given CB154. These rates were related to decreased activities of acetyl coenzyme A carboxylase, fatty acid synthetase, and a-lactalbumin. For all mammary biochemical parameters, cows given CB154 plus PRL had values equal to those of controls. We conclude that periparturient secretion of PRL is essential for maximal synthesis of milk in the postpartum period and that PRL plays a critical role in mammary differentiation of key biochemical steps involved in synthesis of milk. {Endocrinology 109: 23, 1981) I N LABORATORY species, PRL is required for initiation and maintenance of lactation (1), but its role in cattle is unclear. For example, reducing concentrations of serum PRL during established lactation does not reduce milk yields in cows (2-4) as it does in nonruminants (5, 6). On the other hand, Karg and Schams (7) reported that cows treated periparturiently with CB154 (2-Br-aergokryptin) exhibited markedly reduced concentrations of plasma PRL and that subsequent milk production was nearly abolished. Johke and Hodate (8) reported that

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