The metabolic effects of oral ingestion of minute quantities of carbohydrate during prolonged starvation were studied in nine obese subjects. Measurements were made during a control period of total starvation, during the ingestion of 7.5 g carbohydrate daily, and finally during the ingestion of 15.0 g carbohydrate daily. Daily ketoacid excretion fell after carbohydrate ingestion and was significantly correlated (r = 0.62, P < 0.01) with the amount of carbohydrate administered. Despite this fall in ketoacids, the concentration of blood ketoacids, plasma free fatty acids, and serum insulin remained constant throughout the study. Urinary ammonium excretion, closely correlated with ketoacid output (r = 0.95, P < 0.001), also fell significantly after carbohydrate ingestion. No significant changes were present in extracellular or urinary pH. Urea nitrogen excretion did not change when urinary ammonium output fell. These results indicate that: the excretion of ketoacids and ammonium in starving man is exquisitely sensitive to minute amounts of ingested carbohydrate; the change in ketonuria appears to be due to increased renal ketoacid reabsorption after carbohydrate ingestion; and the nitrogen-sparing effect of reducing renal ammonium output in starvation can be dissociated from nitrogen sparing occurring because of changes in urine urea excretion.