Abstract: Substituted 4,5-dihydro-6-(substituted)phenyl-3(2H)-pyridazinone compounds and 6-(substituted)phenyl-3(2H)-pyridazinone compounds and pharmaceutically acceptable salts thereof are useful as cardiotonic and antihypertensive agents.Said compounds cause a significant increase in myocardial contractility in the dog. Said compounds also cause a decrease in blood pressure in the spontaneously hypertensive rat. Said compounds are produced by reacting substituted .gamma.-oxobenzenebutanoic acids with suitably substituted hydrazines to provide 4,5-dihydro-6-(substituted)phenyl-3(2H)-pyridazinones which are dehydrogenated to 6-(substituted)phenyl-3(2H)-pyridazinones.Both the intermediate 4,5-dihydro-6-(substituted)phenyl-3(2H)-pyridazinones and the 6-(substituted)phenyl-3(2H)-pyridazinones are useful as cardiotonic and antihypertensive agents.