SUMMARY Changes in the composition of the aqueous phase of milk were found to precede oestrus by up to 4 days in five out of six goats. However, the composition had returned to normal by the time oestrous behaviour was first observed. The alteration in composition was characterized by increases in milk sodium and chloride concentrations, and decreases in potassium and lactose; fat, protein and immunoglobulin concentrations were unaffected. These changes were also observed in autotransplanted (i.e. denervated) glands. Large doses of exogenous oestrogens (to match the secretion rate in late pregnancy) administered to goats in anoestrus resulted in a significant fall in milk yield, increases in milk fat, protein, immunoglobulin and sodium concentrations, and a decrease in potassium. Chloride and lactose concentrations were not significantly changed except in some goats when the milk yield was severely affected 4 days after oestrogens were given i.m. Possible hormonal mechanisms and the nature of the changes in the secretory epithelium before oestrus and as a result of oestrogen treatment are discussed.