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The technique of perfusion of the isolated udder is likely to give valuable information on the mechanisms of milk formation, provided it can be shown that most of the milk collected is actually a product of the secretory activity of the gland during the experiment. This point is not easy to demonstrate by ordinary chemical methods, because present-day techniques only permit of a rather limited perfusion period, and also because in the relatively large quantity of tissue involved the various precursors, intermediates and products are distributed in an unknown manner. Moreover, although some of the milk held in the alveoli and finer branches of the duct system can be collected via the teat after administration of oxytocin, the difficulty remains that the proportion of preformed to newly formed constituents in any milk sample must remain uncertain. Two modern methods for perfusing the isolated bovine udder have so far been described (Petersen, Shaw & Visscher, 1939; Peeters & Massart, 1947), and a number of studies, mainly on the physiology of milk ejection, have been carried out by means of these techniques. The demonstration that the udder of the intact goat synthesizes milk fatty acids from 14C-labelled acetate (Popjak, French & Folley, 1950, 1951) suggested the possibility of a direct test of the ability of the isolated preparation to synthesize milk constituents in the normal manner. The results presented in this paper show that the perfused, isolated udder can use CH314COONa added to the perfusion fluid for synthesis of milk constituents, in particular fatty acids. They show further, however, that the conditions of perfusion need to be most carefully controlled if comparisons are to be made with processes occurring in the living animal. For this reason the interpretation of the results, in terms of the synthetic mechanisms occurring in the isolated organ, must be regarded as tentative at present. EXPERIMENTAL

Synthesis of milk fat from acetic acid (CH314COOH) by the perfused isolated bovine udder