Because zinc apparently has a role in amino acid utilization, we have studied possible consequences of zinc deficiency on the metabolism of amino acids in the intestinal mucosa by examining proteins transported via the mesenteric lymph in zinc-deficient and zinc-supplemented rats. Mesenteric lymph ducts were cannulated under pentobarbital, and on recovery the animals were fed [14C]leucine as a pulse dose by duodenal infusion. Protein output via intestinal lymph was generally constant over the infusion range studied. Total lymph protein output was not affected by zinc deficiency or supplementation. The electrophoretic pattern of the lymph protein revealed a protein band that migrated with the alpha2-globulins coincidental to zinc deficiency; the band disappeared after zinc supplementation. There was an inverse zinc-copper relationship in mesenteric lymph; duodenal infusion of zinc to deficient rats was followed by a decrease in lymph copper. Deficiency of zinc seems to have little influence on the quantitative aspect of protein carried in mesenteric lymph but seems to be involved with the nature of the protein in this body fluid.