While it is known that efficient lactation depends upon many hormones it is not so well known whether all act directly on the mammary glands, and the effects of fluctuation in the circulating concentrations of hormones on intact animals have been little studied. Recent work on the mechanism of secretion of the aqueous phase of milk has suggested the site of active monovalent ion transport (Linzell & Peaker, 1971a, b). It is therefore important to know if aldosterone affects these processes in normal animals. Lactating Saanen or Saanen-Welsh cross-bred goats were used. They were milked every day at approximately 09.30 and 16.30 h and the milk yield of each gland and the time of milking recorded. In addition, milk samples were taken every day at the afternoon milking for 4 days before, and during treatment. The techniques and analytical procedures have been described previously by Linzell & Peaker (1971