1. Injection of citrate or EGTA solutions into the lumen of the mammary gland of goats in quantities sufficient to reduce ionized calcium to less than one-tenth of normal, led to increases in milk concentrations of Na and Cl and decreases in K and lactose. 2. Subsequent milk yields were decreased in glands treated with citrate but not in those treated with EGTA. 3. Blood-milk potential difference decreased (i.e. towards zero) in glands in which citrate was present. 4. In goats milked hourly with the aid of oxytocin, milk Na and Cl concentrations increased while K and lactose decreased; there was no apparent decrease in Ca2+ concentration. 5. It is suggested that ionized calcium in milk is essential to preserve the integrity of the mammary epithelium during lactation.