We hypothesized that O₂ wasting of Ca²⁺ handling in the excitation-contraction coupling in ryanodine-treated failing hearts might derive from an increased external Ca²⁺ extrusion via Na⁺/Ca²⁺ exchanger and futile Ca²⁺ cycling via sarcoplasmic reticulum (SR) Ca²⁺-ATPase. We tested this hypothesis by mechanoenergetic studies using rat left ventricular slices. After the slices were treated with ryanodine (0.1 µM), 1-Hz free shortening significantly decreased by 78-85%, whereas the observed O₂ consumption (O₂) required for total Ca²⁺ handling, increased from 0.79 to 1.13 ml O₂ · min¹ · 100 g LV¹ (155.6% of control). We reconfirmed that cyclopiazonic acid (10 µM), a blocker of SR Ca²⁺-ATPase, decreased O₂ by 75-80% in normal slices. However, 100 µM of cyclopiazonic acid was needed to inhibit the O₂ by 80% after ryanodine treatment. Blockade of a sarcolemmal Na⁺/Ca²⁺ exchanger by KB-R7943 (10 µM) significantly decreased O₂ by 45% after ryanodine treatment without significant effects on normal slices. Our results indicated that the O₂ increase following ryanodine treatment was derived from a net change of an increased external Ca²⁺ extrusion via Na⁺/Ca²⁺ exchanger and futile Ca²⁺ cycling via SR Ca²⁺-ATPase.