Thyroid adenomas and carcinomas were induced after continuous (12--15 months) treatment of Wistar rats with methylthiouracyl. Ultrastructural cytochemical technique was used in studying the localization of thyroid peroxydase in different follicular cell structures. In control animals, the enzyme was localized on the ER membranes, the internal lamellae of Golgi's apparatus, the perinuclear membrane, the microvilli and in isolated apically arranged lisosomes and microvesicles. No peroxidase activity was detected in animals, having received one-month methylthiouracyl treatment. This was confirmed also by the chromatographic study, showing the presence of inorganic iodine only. Peroxidase activity was demonstrable in only part of the thyroid adenomas and carcinomas; the enzymes was localized in the same subcellular structures as in control animals. In these tumors a variety of iodine-containing components were also identified, largely of the MIT and DIT group.