The relationship between plasma and brain amino acids and short-term food intake after administration of albumin, or its constituent amino acids, was examined. Rats given protein (0.85 g chicken egg albumin) or an amino acid mixture patterned after egg albumin reduced their food intake during 1 h of feeding beginning 30 min after gavage. Similarly, when given separately, the essential (EAA) and nonessential amino acid (NEAA) fractions of egg albumin caused comparable decreases in food intake. As the dose increased from 0.5 to 1.5 g the duration of anorexia prolonged to 12 h. Little change occurred in plasma amino acids at 30 and 60 min after albumin at 0.85 g, although many increased by 25-50% at 60 min after 1.5 g. Marked changes in plasma occurred after gavage with the total mixture of constituent free amino acids and after either EAA or NEAA fractions. Brain amino acid concentrations were little affected by albumin and did not show consistent changes after the amino acid treatments. Thus the reductions in food intake after ingestion of albumin or of its constituent amino acids were not predicted from the resulting changes in either plasma or brain concentrations of amino acids.