Sodium DL-3-hydroxybutyrate was administered to obese subjects (more than 150% ideal body-weight) who were either receiving a 2 . 5 MJ (600 kcal) diet containing 34 g protein on one day with a total fast (water and vitamins only) on the next day, for 21 days, or were undergoing therapeutic starvation for 14 days. Both intravenous and oral hydroxybutyrate significantly reduced net body protein loss as measured by total urinary nitrogen and 3-methylhistidine excretion. Hydroxybutyrate administration did not significantly affect the rate of weight-loss but seemed to increase the fat/lean ratio of the tissue loss. The subjects experienced no untoward effects and none complained of hunger while receiving 3-hydroxybutyrate.