To assess muscle breakdown during avian coccidiosis, the level of the nonmetabolizable amino acid 3-methylhistidine (3MH) was determined in muscle and plasma from chickens infected with the cecal parasite Eimeria tenella. The change in 3MH level during infection was determined in birds, each inoculated with 0 to 200,000 sporulated oocysts. The effect of levels of parasitism was evaluated at 6 d postinoculation. The 3MH levels of plasma and muscle were determined by HPLC after derivatization with fluorescamine. Weight gains, packed cell volumes, and gross lesion scores were also determined. E. tenella infected birds with lesion scores of 3 or 4 had significantly elevated plasma and muscle 3MH, whereas infected birds with lesion scores of 0, 1, or 2 did not have elevated plasma and muscle 3MH; however, there was a linear inverse relationship between weight gain and both plasma and muscle 3MH. The results suggested that muscle breakdown, as assessed by plasma and muscle levels of 3MH, was elevated during the acute stage of E. tenella infection and was most likely associated with anorexia caused by infection. However, the correlation of 3MH levels with severity of infection was not as strong as that previously observed for E. acervulina infection, most likely due to the differences in pathology caused by the two species.