A study was carried out on the reaction of 4-hydroxy-1-phenyltetrahydropyrimidine-2(1H)-thione and its 6-methyl analog with a series of functionally substituted hydrazines and amines. Substitution of the hydroxyl group leads to 4-substituted tetrahydropyrimidine-2(1H)-thiones. Partial recyclization is observed in solutions of the reaction products with thioaroylhydrazines and a ring-ring tautomeric equilibrium between tetrahydropyrimidine-2(1H)-thiones and 2,3-dihydro-1,3,4-thiadiazoles is established.