Four experiments were undertaken to establish the effects of diet and feeding regimen on serum amino acid concentrations of growing swine. Single meal feeding resulted in fluctuations in serum amino acid levels that could not be attributed solely to diet. A short-interval feeding regimen was proposed that resulted in relatively stable serum amino acid concentrations during the refeeding program. This procedure was suggested to be useful in attempting to evaluate protein quality by examination of serum amino acid fluctuations. A semipurified diet was observed to result in extremely high serum levels of methionine and several other amino acids, raising the possibility that the dietary amino acid requirements of swine fed semipurified diets were lower than accepted values. When the dietary methionine concentration was altered from 0.41% to 0.70%, an adaptation period of about 3 days was required to stabilize serum methionine at a new level.