Effects of administration of thiosemicarbazide (TSC) and L-tyrosine supplemented to the diets on tenderness and intramuscular collagen levels in muscles of 10 months old female quails (Cortunix cortunix j.) were studied. Forty birds were alloted into 4 dietary treatments, i.e. Basal diet, B.D.+tyrosine (1.5%), B.D. + TSC (0.03%) and B.D. + tyrosine (1.5%)+TSC (0.03%). All diets fed for 18 days were made isocaloric and isonitrogenous by supplementing tyrosine at the expense of L-glycine. The semipurified basal diet was composed of soybean oil meal as the only source of nitrogen, glucose and corn oil as major ingredients. Birds fed the diets containing TSC alone or along with tyrosine consumed significantly less feed and showed severe body weight loss compared to the birds fed the other diets (p$lt;0.05). Administration of either TSC or tyrosine, however, did not induce significant changes in weight of organs such as liver, heart, spleen and thyroid gland expressed as % of body weight. No statistical differences were observed in plasma ascorbic acid contents, collagen levels in thigh muscle (biceps femoris) and levels of thermal labile collagen in breast muscle (pectoralis superficialis) among the birds fed the various diets. Shear values measured by an Instron tended to be higher in the order of control $lt; tyrosine $lt; TSC Tyrosine + TSC groups without significance. Collagen content in breast muscle appeared significantly higher in birds fed the diets with TSC or tyrosine + TSC (P$lt;0.05). Percentage of thermal labile collagen of total collagen was reduced by administration of TSC alone. The tendencies appeared here are partly not in agreement with those observed in chicken. It is not clear whether the results observed in present experiments were due to the severe weight loss caused by TSC or to species differences.