Novel 2-arylbenzimidazoles and azabenzimidazoles were synthesized, and their inotropic action was evaluated. Changes in left ventricular pressure, dP/dt max, were measured as an index of cardiac contractility. The structural features that impart optimal inotropic activity are presented. The most potent compounds were evaluated orally in conscious dogs with implanted Konigsberg pressure transducers. To investigate the mechanism of action, the most potent compounds were tested for their calcium-sensitizing properties and their potential for the inhibition of phosphodiesterase. Two compounds, 1 and 41, showed interesting in vitro and oral activity without side effects. They have a more potent calcium-sensitizing effect than MCI-154 and are under further investigation.