In an attempt to eludicate the histogenesis of thyroid carcinoma, histopathological and immunohistological studies were conducted on rat thyroid tumors induced by diisopropanolnitrosamine (DIPN). A total of 235 nodular lesions detected on serial sections of whole thyroid glands were classified into three types as follows: foci of cellular alteration at the single follicle level (type 1); multifollicular nodules considered to be malignant or potentially malignant (type 3); and multifollicular, proliferative nodules other than type 3, including both hyperplastic and neoplastic nodules (type 2). Analysis of the frequency of nodular lesions after various periods of time following exposure to DIPN revealed that the size of the nodules, their pattern of development, and their growth activity, as measured by the labeling index for bromodeoxyuridine (BrdU), showed sequential changes. Type 1 lesions appeared early; these lesions were the smallest and were considered to indicate the initial morphological change. Subsequently, type 2 nodules of intermediate size and labeling index, and finally type 3 nodules with the largest size and the highest labeling index developed; the latter type 3 nodules become more numerous during the latest phase following DIPN exposure. These observations support a multistage hypothesis of tumorigenesis.