A total of 630 8-week-old female Hybrid Converter turkeys were divided (based on their body weights) into 6 groups, with 7 replicates per group and 15 birds per replicate. All birds were fed identical isocaloric and isonitrogenous wheat-soybean meal-based diets without (group 1) or with (groups 2 to 6) increasing levels of supplemental methionine (Met). The total content of Met in diets 1 to 6 was as follows (%): 0.29, 0.32, 0.40, 0.47, 0.56, and 0.61 at 9 to 12 wk of age and 0.24, 0.28, 0.34, 0.42, 0.47, and 0.55 at 13 to 16 wk of age. In both feeding phases, dietary Met levels in group 3 corresponded to those recommended by the National Research Council (NRC) (1994). Different dietary Met concentrations had no influence on feed intake, the final body weights of turkeys or carcass dressing percentage. Only in the first experimental feeding period (9 to 12 wk), the lowest dietary Met content significantly deteriorated the feed conversion ratio (FCR), whereas the highest Met content led to a significant improvement in FCR. After 8 wk of experimental feeding, dietary treatment 1 contributed to a significant increase in the activity of catalase (CAT) (blood and breast muscles) and superoxide dismutase (liver), an increase in lipid peroxides concentrations (blood, breast muscle) and a decrease in total glutathione (GSH+GSSG) content (breast muscles), in comparison to treatment 3 which is comparable to NRC recommendations. The highest level of dietary Met significantly increased blood total antioxidant potential (FRAP) values and glutathione content in the liver. To sum up, in the final feeding period between 9 and 16 wk of age, the growth performance of female turkeys was not deteriorated by dietary Met deficiency or excess (-30% and up to +50% relative to NRC recommendations, respectively). The total antioxidant potential can be effectively increased by dietary Met supplementation, but the highest Met level may lead to unbalanced oxidative changes in the body as indicated by lower FRAP values and a lower GSH/GSSG ratio in the liver.