The effects of food deprivation for up to 24 hours on plasma metabolic parameters in the rat have been studied. Liver dry weight and glycogen content dropped significantly from 3 hours of food deprivation onwards. Total muscle glycogen supplied about as much glycosyl residues or precursors as did the liver. Plasma glucose, urea, lactate and total and essential amino acids decreased significantly from 3 hours of fasting onwards. Glycerol, free significantly from 3 hours of fasting onwards. Glycerol, free fatty acids, beta-hydroxybutyrate and acetoacetate showed significant increases with fasting. Alanine, serine, arginine, threonine, aspartate plus asparagine and proline showed significant decreases with fasting. Several other amino acids showed almost no change with fasting. Lysine, leucine plus isoleucine and taurine showed biphasic changes in their concentrations with a minimum at 6 hours and a transient recovery at 12 hours of fasting Essential amino acids decreased more than the non essential ones. With fasting there is a shift in ammonia disposal with lower urea concentrations as nitrogen is better conserved. The results seem to suggest that there is a constant release of substrates, through liver and peripheral tissue proteolysis, that is counteracted by differential utilization of amino acids during fasting.