A series of 4,5-dihydro-6-[4-(1H-imidazol-1-yl)phenyl]-3(2H)-pyridazinones and related compounds were synthesized and evaluated for positive inotropic activity. Most members of this series produced dose-related increases in myocardial contractility that were associated with relatively minor increases in heart rate and decreases in systemic arterial blood pressure. Introduction of a methyl substituent at the 5-position of 1 (CI-914) produced the most potent compound in this series (11, CI-930). Compound 1 is more potent than amrinone whereas compound 11 is more potent than milrinone. The inotropic effects of 1 and 11 are not mediated via stimulation of beta-adrenergic receptors. Selective inhibition of cardiac phosphodiesterase fraction III represents the principal component of the positive inotropic action of 1 and 11.