An experiment was performed to determine the effectof sodium deprivation on pituitary and plasma radioimmunoassayable proiactin, growth hormone and thyrotropin, aldosterone and total corticosteroid secretory rates, plasma corticosterone, pineal, adrenal and body weights, serum sodium and hematocrit. Compared to the results in control animals, sodium deprivation pro-duced increased aldosterone secretory rates, increased plasma and pituitary proiactin levels, lower serum sodium, higher hematocrit, and lower body weight. The evidence is interpreted as indicating that pituitary proiactin secretion is essential in enhancing the aldosterone secretory response to sodium deprivation.