For the examination of changes in the mammary blood flow in cows during the lactation period it is desirable to relate the blood flow to milk yield as well as to udder weight. Therefore, a method is required which will permit repeated determinations of the true udder weight in live animals. In goats and humans several authors have measured mammary volume by displacement of water and from alginate or plastic mesh moulds, and the accuracy of each method has been determined (see Linzell 1966). Reinecke (1925) and Filipovic (1928) made a few measurements by water displacement in cows, and Linzell (1966) made a wire and plastic mesh mould impregnated with alginate, but the accuracy has not been checked in cows. The following is a report on a practical method of making a mould of the udder of conscious cows and on its accuracy by comparison with the weight of the udder at death.