Fischer 344 rats (810 of each sex) were divided into treatment groups and fed diets containing 0, 10, 40, 600, 1200, or 2400 ppm sulfamethazine. Serum samples were analyzed for levels of thyroid-stimulating hormone (TSH), total thyroxine (T4), total triiodothyronine (T3), and T3 uptake after 12, 18, or 24 mo of continuous dosing. There were no statistically significant differences in T3 levels or percent T3 uptake for either sex after any of the exposure periods. The serum T4 levels were lower (p less than 0.05) for females dosed at 1200 and 2400 ppm for 18 mo and for males dosed at 600, 1200, or 2400 ppm sulfamethazine for 24 mo than for those dosed at levels of 40 ppm or less. Serum TSH levels showed a general increasing trend (but not statistically significant) among animals receiving 600 ppm or more sulfamethazine. There was a significant dose-related reduction in (T3 + T4)/TSH ratio for both sexes (p less than 0.05) after 18 and 24 mo of exposure at dose levels of 600 ppm or more. A lack of response at 12 mo may have been due to the shorter treatment time. At each sacrifice period both sexes of rats fed sulfamethazine at 1200 and 2400 ppm had significantly heavier (p less than 0.05) thyroid weights than animals fed control diet. The heavier thyroid weights in the dosed animals may have resulted from increased TSH levels. The cause of reduction in serum T4 was not clearly evident. Therefore, the thyroid hormone to pituitary feedback mechanism apparently compensated for sulfamethazine effects in most animals. This would suggest that the thyroid gland was not irreversibly affected.