Fractional protein synthesis rates (FSR) were determined in lambs fed fresh sulla (Hedysarum coronarium) during an established Trichostrongylus colubriformis infection. Forty-eight days after infection, the FSR in the duodenum, ileum, mesenteric lymph nodes, spleen, liver, thymus, muscle and skin was determined. Parasite infection increased the FSR in the smooth muscle of the duodenum and ileum and in the mesenteric lymph nodes. This suggests an increase in amino acid requirement for the synthesis of constitutive proteins in these tissues. However, an established parasite infection did not result in the diversion of amino acids from muscle to the small intestine nor the liver.