A total of 8 medullary carcinimas, 134 adenomas of C-cells, 32 solid focal over-growths of C-cells, and 124 normal thyroid glands of rats were investigated by histochemical, immunomorphological and histological methods. Modullary carcinomas were characterized by a high enzymes of hydrolysis of ethers of carboxylic acids, tissue respiration, and by a moderate or low activity of enzymes of the initial link of glycolysis, oxidative deamination, synthesis of amino acids, hydrolysis of monoethers of orthophosphoric acid, pentose and citric acid cycles. In medullary carcinomas, as well as in normal C-cells and adenomas of C-cells, there was revealed the phenomenon of occult metachromasia with the absence of a microsomal tissue-specific antigen and direct reaction to amyloid. The authors consider medullary carcinomas to be analogues of solid carcinoma with amyloidosis of the stroma of the human thyroid gland, differing from the latter by specific metabolic reactions, and therefore, they may serve as an experimental model in studies of these diseases.