Heterocyclic substituted indolinones of the formula: ##STR1## wherein R.sub.1 is a hydrogen atom or an alkyl or hydroxyalkyl radical, R.sub.2 is a carboxyl group or an alkoxy-carbonyl, hydroxyalkyl or alkylcarbonyloxyalkyl radical or R.sub.1 and R.sub.2, together with the carbon atom to which they are attached, form a lactone having 4, 5 or 6 ring atoms, A-B is --CH.sub.2 --CH(R.sub.3)--, --CH.dbd.CR.sub.3 --, --CH.sub.2 --NH--, --NH--CH.sub.2 --, --CH.sub.2 --O--, --O--CH.sub.2 --, --CH.sub.2 --S-- or --S--CH.sub.2 -- radical, R.sub.3 being a hydrogen atom or an alkyl or hydroxyalkyl radical, or A is a valency bond and B is a methylene radical ##STR2## or an imino group ##STR3## R.sub.4, R.sub.5, and R.sub.6 independently being hydrogen atoms or alkyl radicals wherein the alkyl groups independently contain 1 to 6 carbon atoms; as well as the tautomers thereof and the physiologically acceptable salt thereof and the physiologically acceptable salts thereof with inorganic and organic acids, are disclosed, such as, for instance, the compound 5-(2,3,4,5-tetrahydro-3-oxopyridazin-6-yl)-3-ethoxycarbonyl-3-methyl-1H-indol n-2-one. The compounds of the invention are useful in the treatment of heart and circulatory diseases, for example, by producing a positive inotropic action in a patient to whom the compounds are administered.