To evaluate the mechanism of the promoting effect of goitrogens on thyroid tumorigenesis, well-known goitrogens having different pharmacologic action, i.e., thiourea, phenobarbital sodium (PB), potassium thiocyanate (KSCN), and 3,4,5,6-tetrachloro-2',4',5',7'-tetraiodo-fluorescein sodium salt (Rose Bengal B, FD&C Red No. 105) (FR105) were administered to the DHPN-initiated and non-initiated F344 male rats in the drinking water for 25 weeks. Remington's iodine deficient diet (I-def) was fed as a positive control. These goitrogens showed significant tumor promoting effect or promoting tendency on the rat thyroids. According to the changes in thyroid morphology and thyroid-related hormone titers observed in the present study, we proposed to classify goitrogens at least into 2 groups, i.e., iodine deficiency-type promoters and the iodine excess-type promoters. The former contains goitrogens inducing TSH-stimulated diffuse goiter composed of uniform follicles with activated tall follicular epithelial cells, such as thiourea, KSCN and PB, and the latter contains goitrogens inducing colloid goiter composed of a mixture of colloid-rich follicles with flat follicular cells and normal-looking follicles with cuboidal follicular cells, such as FR105. This classification may be useful for the risk assessment of goitrogens.