obtained. It was found that the reaction course depends on the purity of the isothiocyanate used and also, in the case of dialkylaminoamines, the self-cyclization occurs. Besides the dialkylamino derivatives of 3-monosubstituted 2-thiohydantoins also new monoalkylamino, amino and heterocyclic derivatives were synthesized. The aryldiazonium derivative of 3-monosubstituted 2-thiohydantoin yielded both