Left ventricular (LV) inotropic and lusitropic responses to a calcium sensitizer, pimobendan, were compared between normal and failing hearts. Heart failure was induced by rapid ventricular pacing in 6 dogs instrumented with a micromanometer and a conductance catheter. The effects of pimobendan were evaluated in the conscious state before and after development of heart failure. Pimobendan dose-dependently increased the slope of the end-systolic pressure-volume (P-V) relation (Ees) in both normal and failing hearts, whereas its magnitude was markedly attenuated in failing hearts. Heart rate (HR) was increased by pimobendan in normal heart but did not change in failing heart. LV relaxation, assessed by peak - dP/dt and the time constant of isovolumic pressure decay (Td), was substantially improved to the same extent in failing and normal hearts. Consequently, Ees and Td exhibited a hyperbolic relation over a wide range of contractility states. In normal heart, pimobendan caused a leftward shift of the diastolic P-V relation while maintaining a similar curve. In failing heart, however, this relation shifted directly downward with a concomitant increase in end-diastolic volume, indicating a reduction in the constraints on LV distention and a resultant increase in preload reserve. Thus, pimobendan accelerated LV isovolumic relaxation and improved distensibility in conscious dogs with tachycardia-induced heart failure despite the marked attenuation of inotropic responses.