The effects of acute administration of tumour necrosis factor-alpha (cachectin) (TNF-alpha) or of malignant tumour growth (Walker-256 carcinosarcoma) on hepatic availability and uptake of individual amino acids were compared. The results show that, in spite of lowering the hepatic availability of alanine, aspartate, serine, glycine and proline, the cytokine increased both the total amino acid hepatic uptake and the individual uptakes of alanine, glutamate, serine, threonine, proline, lysine and arginine, while decreasing those of leucine, isoleucine and phenylalanine. Tumour burden resulted in an increase in the hepatic availability of glutamine, threonine, glycine, lysine, leucine, isoleucine, valine and phenylalanine. Total liver amino acid uptake was unaffected, whereas the individual uptakes of alanine, threonine and proline were increased and those of glutamate, glutamine, serine and leucine were decreased. When effects of the cytokine are compared with those induced by tumour growth, there are similar increases in net utilization for alanine, proline and leucine, and a 3-fold difference in the increase observed for threonine. Unmatched effects are seen for glutamate, glutamine, aspartate, glycine, lysine, arginine, valine, phenylalanine and serine.