For further investigation of the mechanism involved in D,L-3 hydroxybutyrate (DL3HB)-induced inhibition of feeding [5], we measured cumulative food intake following subcutaneous injections of DL3HB or D-(-)-3-hydroxybutyrate (D3HB), and plasma ketone levels following subcutaneous injections of D3HB or acetoacetate (ACAC) in rats. Subcutaneously injected DL3HB (10 mmoles/kg body weight) and D3HB (5 mmoles/kg body weight) reduced food intake a similar amount. After injection of D3HB or ACAC (5 mmoles/kg body weight) plasma concentrations of the injected ketone bodies reached a similar high level at 20 min and then decreased with time (40 and 60 min). Even though ACAC failed to reduce food intake, injected ACAC disappeared faster from the circulation than injected D3HB. The data are consistent with the idea that oxidation of injected D3HB to ACAC reduces food intake in rats.