A series of [4-(substituted-amino)phenyl]pyridazinones and [4-(substituted-methyl)amino]phenyl]pyridazinones was synthesized and evaluated for inotropic activity in vitro and for cardiohemodynamic effects in vivo. Above all, 6-[4-(4-pyridylamino)phenyl]-4,5-dihydro-3(2H)-pyridazinone hydrochloride (4, MCI-154) and 6-[4-(4-pyridylamino)phenyl]-5-methyl-4,5-dihydro-3(2H)-pyridazinone hydrochloride (5) showed extremely potent positive inotropic activity along with vasodilating activity. Regarding dP/dtmax (an indicator for cardiac contractility), ED30's (doses that increased dP/dtmax by 30%) of compounds 4 and 5 were 8.5 +/- 1.9 and 4.4 +/- 0.6 micrograms/kg, respectively, where that of amrinone was 471.9 +/- 94.1 micrograms/kg. Structure-activity relationships of these series are presented, and a plausible model of receptor binding is discussed.