A series of 3-alkyl-4-(arylidenamino)-4,5-dihydro-lH-l,2,4-triazol-5-ones, 6-10, was obtained from the reaction of the corresponding 3-alkyl-4-amino-4,5-dihydro-lH-l,2,4-triazol-5-ones, 1, with the appropri-ate aldehydes. 3-alkyl-4-(alkylamino)-4,5-dihydro-lH-l,2,4-triazol-5-ones of type 11-16 were synthesized from the selective reduction of compounds 5-10 with NaBH4 in diglyme or DMSO medium as a general and convenient method.