Six groups of female outbred Wistar rats were subjected to the following treatments: 2,500 ppm 3-amino-1H-1,2,4-triazole (amitrole) administered in the drinking water (group I); partial thyroidectomy and administration of amitrole in the drinking water (group II); partial thyroidectomy plus autoimplantation of resected thyroid tissue plus administration of amitrole in the drinking water (group III); no treatment (control of group I) (group IV); partial thyroidectomy without amitrole administration (control of group II) (group V); or partial thyroidectomy plus autoimplantation of resected thyroid tissue without amitrole administration (control of group III) (group VI). Goiters developed in all rats given amitrole (groups I-III). Invasive growth of the proliferating follicular tissue through the capsule into adjacent fatty tissue and of pericapsular blood vessels was frequent in these groups (P less than 0.001); invasive lesions were more frequent in the lobectomized thyroid glands of rats (groups II and III). Seven papillary adenomas composed of atypical cells with hyperchromatic nuclei and many mitoses were found in the thyroid glands and grafts in amitrole-treated groups I-II. No invasive growth or adenomas appeared in the control rats of groups IV-VI. Cholangiofibrosis developed in 5 of 100 rats in groups I-III.