The free amino acids in the plasma of normal and Trypanosoma vivax infected sheep and also in the trypanosome organism per se were determined. Threonine, asparagine, methionine, tyrosine, citrulline, ornithine, histidine, arginine and urea were significantly decreased in the plasma of infected sheep. Alanine was significantly elevated in plasma of infected sheep. Valine, isoleucine, leucine values were also elevated in the plasma of infected sheep, although not at significant level. Taurine was present among the amino acid profile of T. vivax organisms and also in the plasma of normal and infected sheep. The significance of the amino acid changes in terms of host and T. vivax metabolism is discussed.