Recently we have reported that fed hypophysectomized rats excrete a water load as rapidly as fed normal rats (Bauman, 1964). It was shown that fasting reduced this ability sharply in the hypophysectomized animals, but not to the same extent in normal rats. Also, both fasting and hypophysectomy impaired the urine concentrating ability of rats. Normal rats fasted for 24 hr. concentrated urine to only about half the osmolality of urine obtained from rats fed ad libitum. It has been shown that rats maintained on low protein diets for prolonged periods have a reduced renal concentrating ability (Dicker, Heller & Hewer, 1946; Heller & Dicker, 1947), but we are unaware of any studies on the effect of acute fasting on this function in experimental animals. To study the effect of short-term fasting further, we gave normal female rats, weighing from 170 to 200 g., a volume of water equivalent to 4·5%