Herbicidal pyrazolin-5-one derivatives of the formula ##STR1## in which R.sup.1 represents alkyl, cycloalkyl, halogenoalkyl, alkenyl, alkynyl, alkenyl or alkynyl substituted by optionally substituted phenyl, halogenoalkenyl, alkoxy, alkoxyalkyl, alkylthioalkyl, alkylsulphonylalkyl, alkylsulphinylalkyl, alkoxycarbonylalkyl, in each case optionally substituted aryl, aralkyl, aryloxyalkyl or arylthioalkyl, an optionally substituted heterocycle, heterocyclylalkyl or the --NH--CO--R.sup.3 or --CO--O--R.sup.4 groups, in which R.sup.3 and R.sup.4 in each case independently of one another represent alkyl or aryl,R.sup.2 represents hydrogen, alkyl, cycloalkyl, halogenoalkyl, alkenyl, alkinyl, halogenoalkenyl, alkoxyalkyl, alkylthioalkyl, alkylsulphonylalkyl, alkylsuslphinylalkyl or alkoxycarbonylalkyl andAr represents optionally substituted aryl or a heterocycle which is optionally substituted and optionally fused,excluding the compounds 1-(3-chlorophenyl)-3-methylthiomethyl-4-(N-hydroxyaminomethylidene)-pyrazoline-5-one, 1-(4-chlorophenyl)-3-methylthiomethyl-4-(N-hydroxyaminomethylidene)-pyrazolin-5-one, 1-(4-trifluoronethylphenyl-3-methylthiomethyl-4-(N-hydroxyaminomethylidene))-pyrazolin-5-one and 1-(4-n-nitrophenyl)-3-methylthiomethyl-4-(N-hydroxyaminomethylidene)-pyrazolin-5-one.