Within the framework of the study of the relationships between the structure and thyreostatic activity of cyclic analogues of thiourea, a series of 5-substituted 3-mercapto-4H-1,2,4-triazoles was prepared in order to answer the question of how the presence of another nitrogen atom in position 1 and the presence of the substituent in position 5 would influence thyreostatic activity. The synthesis was based on 1-acylsubstituted thiosemicarbazides which were cyclized to the pertinent 3-mercapto-4H-1,2,4-triazoles. Furthermore, esters of dicarboxylic acids were used, which with thiosemicarbazide in alcoholate yielded the pertinent mercaptotriazoles. Selected 5-substituted 3-mercapto-4H-1,2,4-triazoles were tested on adult male rats of Wistar strain. The effects of the agents under study on the level of serum thyroxine (T4), total body weight, heart weight, thyroid gland weight, and the number of leucocytes were investigated. The results of the tests show that whereas the presence of nitrogen in position 1 influenced the thyreostatic activity of the agents prepared in an insubstantial manner only, the presence of a substituent in position 5 acts, except a methyl or ethyl group, dystherapeutically, and with the increasing volume of the substituent therapeutic activity disappears regardless of the fact whether an aliphatic or aromatic substituent is concerned.