Changes in polyamine contents and in the activity of polyamine biosynthetic enzymes were studied in the rat thyroid. Both exogenous and endogenous TSH promptly stimulated the activity of thyroid ornithine decarboxylase (ODC) and S-adenosyl-L-methionine decarboxylase. Treatment of the animals with antithyroid drugs resulted in an increase in circulating TSH level, the activities of the enzymes and thyroidal contents of putrescine, spermidine and spermine. Theophylline, which stimulated methylthiouracil (MTU)-induced goitrogenesis without increasing circulating TSH level, increased the thyroid ODC activity. In contrast, treatment with excess iodine (KI) which induced thyroid involution without affecting circulating levels of TSH, T4 and T3, resulted in a rapid reduction in the thyroid ODC activity. The MTU-induced goiter involuted after MTU withdrawal and the thyroid ODC and polyamine contents decreased within a few days after its withdrawal. The activity of thyroid arginase was increased by MTU and decreased by T4 injection. The enzyme may have some regulatory role in polyamine biosynthesis by controlling ornithine supply. Polyamine levels were elevated in both papillary and follicular adenocarcinomas of the human thyroid. The results suggest that the measurement of thyroid polyamines may be useful for the diagnosis of carcinomas.