Milrinone, a phosphodiesterase 3 (PDE3) inhibitor, is known to enhance left ventricular (LV) contractility by an inhibition of the breakdown of cAMP through the mechanism inhibiting PDE3. However, it is unclear whether milrinone also exerts positive lusitropy, like dobutamine. Here, we assessed the effects of milrinone on in vivo LV relaxation, as well as the Ca²⁺-ATPase activity and the Ca²⁺ uptake function of the cardiac sarcoplasmic reticulum (SR), compared with the effect of dobutamine on those functions. After dobutamine (3 µg · kg¹ · min¹) was administered, the peak value of the first derivative of LV pressure (+dP/dt) increased by 46%, whereas the time constant () of LV pressure decay decreased by 6.9%, respectively. After milrinone (10 µg/kg) was administered, the peak +dP/dt increased to a similar extent as dobutamine (46%), whereas  decreased much more than dobutamine (19.9%; P < 0.05). In LV crude homogenate, the thapsigargin-sensitive, Ca²⁺-ATPase activity-cAMP relationships was significantly less increased by milrinone compared with dobutamine (P < 0.05), indicating the higher sensitivity of the SR Ca²⁺-ATPase activity on cAMP by milrinone than by dobutamine. In the SR vesicles purified from LV muscles, the addition of cAMP increased the SR Ca²⁺ uptake in a dose-dependent fashion, and the PDE3 inhibitors (milrinone and cGMP) significantly augmented this response (P < 0.05). Hence, milrinone substantially improved LV relaxation in association with an acceleration of the SR Ca²⁺-ATPase activity and the SR Ca²⁺ uptake. This acceleration might be due to an inhibition of the membrane-bound PDE3 in the SR, leading to a local elevation of cAMP.