Reduced thyroid hormone concentrations (T4 and/or T3) and increased thyroid-stimulating hormone (TSH) have been proposed to mediate the thyroid tumor promoting effects of hepatic microsomal enzyme inducers (MEI) and antithyroid drugs. TSH is known to stimulate thyroid gland function and growth, as well as neoplasia. Thyroid weight has been used as an indicator of thyroid gland growth in MEI studies, but little is known about the effects of these inducers on thyroid cell proliferation. Therefore, we determined the time-course of thyroid cell proliferation of rats treated with MEI, and with the antithyroid drug propylthiouracil (PTU). Male Sprague-Dawley rats were fed either a basal diet or a diet containing phenobarbitol (PB) (1200 ppm), PCN (500 ppm), or PTU (30 ppm) for 3, 7, 14, 21, 30, 45, 60, or 90 days. PB and PCN treatments did not affect T3, but PTU reduced T3 60%. PB and PCN treatments reduced T4 25%, whereas PTU treatment reduced T4 90%. PB and PCN treatments increased thyroid weight 80%, and PTU increased thyroid weight 500%. TSH was not appreciably altered in PB-treated rats, but was increased 75% and 830% in PCN- and PTU-treated rats, respectively. Thyroid cell proliferation was increased 260, 330, and 850% in rats treated with PB, PCN, or PTU, respectively, for 7 days, but returned to control levels by the 45th treatment day. In conclusion, treatment with MEI that produced mild increases in TSH resulted in dramatic increases in thyroid cell proliferation, which peaked after 7 days of treatment and then returned to control values. This result is similar to that of antithyroid drugs, which produce large increases in TSH. These findings may have important implications for the role thyroid follicular cell proliferation has in mediating the thyroid tumor promoting effects of MEI.