The study of the activity of the constitutive form of nitric oxide synthase (cNOS) revealed that in the papillary thyroid carcinomas it corresponded to that detected in unchanged extratumoral tissue, while the enzyme activity in follicular carcinoma was half lesser. At the same time, the activity of inducible nitric oxide synthase (іNOS) was higher in the papillary and follicular carcinomas. Such changes in the enzyme activity were associated with an increase in its level in papillary carcinomas, and with minor changes in follicular carcinomas. In medullary carcinomas the parameters under study corresponded to those in unchanged tissue, and in the papillary carcinoma metastases without changes in enzyme activity of nitric oxide formation, the level of the latter was much higher. Elevated levels of nitric oxide and іNOS activity in papillary thyroid carcinomas did not depend significantly on the aggression characteristics of the latter, being however absent in tumors of T4 category on a background of reduced cNOS activity and less expressed in tumors surrounded by the tissue in the presence of a chronic thyroiditis. Furthermore, in the papillary carcinomas of papillary or follicular structure nitric oxide level did not differ from the normal range, being slightly higher in tumors of solid or heterogeneous structure with presence of solid areas, whereas in carcinomas of papillary-follicular structure it was twice, and in tissue of solidinsular structure three times higher. іNOS hyperactivity was observed in the carcinomas of different structure, except for tumors of solid structure, in the tumor of which enzyme activity was within the normal range, and in tumor of solid-insular structure where it was significantly higher (as well as cNOS activity) compared with tumors of other structure. Nitric oxide generating system is involved in the transformation of thyroid cells and progression of tumor growth, including through apoptosis regulation, as shown by the results of an analysis of data obtained both in the present study and previously. The nature of such involvement in papillary thyroid carcinomas with different histological structure is different.