Light stabilizers for synthetic resin compositions and synthetic resin compositions containing such stabilizers are provided, comprising(1) at least one hindered heterocyclic amine having the formula: ##STR1## wherein: X is a bivalent linking radical forming with the ring structure ##STR2## a nitrogen-containing heterocyclic ring having from five to seven ring atoms, preferably piperidine;R.sub.1 and R.sub.2 are each selected from the group consisting of lower alkyl, preferably methyl, and R.sub.1 and R.sub.2 taken together as pentylene (CH.sub.2).sub.5 ;R.sub.3 and R.sub.4 are each selected from the group consisting of lower alkyl, and R.sub.3 and R.sub.4 taken together as one of butylene--(CH.sub.2).sub.4, pentylene--(CH.sub.2).sub.5, and CH.sub.2 --C(CH.sub.3).sub.2 --NH--C(CH.sub.3).sub.2 --CH.sub.2 --;n is 1 or 2; andwhen n is 1, Y.sub.1 is selected from the group consisting of hydrogen, O.sup..cndot., OH, alkyl, alkenyl, alkynyl, aralkyl and acyl having from one to about thirty carbon atoms; andwhen n is 2, Y.sub.1 is selected from the group consisting of alkylene, alkenylene, alkynylene, cycloalkylene and aralkylene having from one to about thirty carbon atoms; together with(2) at least one heterocyclic acid hydrazide having the formula: ##STR3## wherein: R is selected from the group consisting of hydrogen, alkyl, aryl and such groups having hydroxyl, carboxylic acid ester, oxyether and carbonyl groups having from one to about thirty carbon atoms; ##STR4## is the residue of a dicarboxylic or tetracarboxylic acid, wherein Z is selected from the group consisting of alkylene, alkenylene, cycloalkylene, alkylcycloalkylene, arylene and alkylarylene having from one to about thirty carbon atoms, and such radicals bearing free carboxylic acid or carboxylic acid ester groups; andm is 1 or 2.In such combinations, the heterocyclic acid hydrazide, which alone is not a light-stabilizer, synergizes the stabilizing effectiveness of the hindered heterocyclic amine.