The hormonal control of lactogenesis has recently been studied by specific chromatographic or enzymic measurements of mammary lactose which have superseded earlier assessments of retained milk or alveolar 'secretory' granules (Chadwick, 1962; Shinde, Ôta & Yokoyama, 1964; Kuhn, 1969). Because lactose is not necessarily representative of other milk solids, protein phosphorus was measured to assess changes in rat mammary casein during lactogenesis.
Wistar-derived rats were used during their first pregnancy or lactation. Mammary tissue was dissected out, pulverized under liquid nitrogen and about 0·8 g was accurately weighed into a round-bottomed glass centrifuge tube. The powder was triturated successively with two 8 ml vols of acetone to remove fat, and with five 8 ml vols of a mixture of chloroform:methanol:conc. HCl (200:100:1, by vol.) to remove phospholipid (Teng, Teng & Allfrey, 1971). After a further extraction with 8 ml acetone the residue was extracted with 8 ml 5% (w/v) trichloroacetic