The present invention relates to novel pyridazinone derivatives of the general formula (I) ##STR00001## wherein A is A1 or A2; ##STR00002## R is unsubstituted or at least monosubstituted C.sub.1-C.sub.10-alkyl, aryl, aryl-(C.sub.1-C.sub.10-alkyl)-, heteroaryl, heteroaryl-(C.sub.1-C.sub.10-alkyl)-, heterocyclyl, heterocyclyl-(C.sub.1-C.sub.10-alkyl)-, C.sub.3-C.sub.10-cycloalkyl, polycycloalkyl, C.sub.2-C.sub.10-alkenyl or C.sub.2-C.sub.10-alkinyl and Ar is unsubstituted or at least monosubstituted aryl or heteroaryl.