Ketosis was produced in sheep by intravenous infusions of large quantities of acetoacetic acid (AcAc). At the infusion rates employed less than 5% was excreted in the urine. Acetoacetic acid-3-C14 was administered also as a continuous infusion to both normal and ketotic sheep. Constant plasma concentrations and specific activities were obtained and with known C14 infusion rates, several parameters of AcAc metabolism were calculated. Mean endogenous AcAc turnover rates were about 0.04 g/hr kg3/4 (2. The percentage of the total respired CO2 derived from AcAc metabolism increased from about 2% in normal ewes to over 30% during ketosis. The results indicate that the metabolism of ketone bodies by the normal ruminant is relatively insignificant but that, during ketosis, their contribution to the animal's total metabolism can be of major importance.