Smaller Ca²⁺ transients and systolic dysfunction in heart failure (HF) can be largely explained by reduced total sarcoplasmic reticulum (SR) Ca²⁺ content ([Ca]_SRT). However, it is unknown whether low [Ca]_SRT is manifest as reduced: (1) intra-SR free [Ca²⁺] ([Ca²⁺]_SR), (2) intra-SR Ca²⁺ buffering, or (3) SR volume (as percentage of cell volume). Here we assess these possibilities in a well-characterized rabbit model of nonischemic HF. In HF versus control myocytes, diastolic [Ca²⁺]_SR is similar at 0.1-Hz stimulation, but the increase in both [Ca²⁺]_SR and [Ca]_SRT as frequency increases to 1 Hz is blunted in HF. Direct measurement of intra-SR Ca²⁺ buffering (by simultaneous [Ca²⁺]_SR and [Ca]_SRT measurement) showed no change in HF. Diastolic [Ca]_SRT changes paralleled [Ca²⁺]_SR, suggesting that SR volume is not appreciably altered in HF. Thus, reduced [Ca]_SRT in HF is associated with comparably reduced [Ca²⁺]_SR. Fractional [Ca²⁺]_SR depletion increased progressively with stimulation frequency in control but was blunted in HF (consistent with the blunted force–frequency relationship in HF). By studying a range of [Ca²⁺]_SR, analysis showed that for a given [Ca]_SR, fractional SR Ca²⁺ release was actually higher in HF. For both control and HF myocytes, SR Ca²⁺ release terminated when [Ca²⁺]_SR dropped to 0.3 to 0.5 mmol/L during systole, consistent with a role for declining [Ca²⁺]_SR in the dynamic shutoff of SR Ca²⁺ release. We conclude that low total SR Ca²⁺ content in HF, and reduced SR Ca²⁺ release, is attributable to reduced [Ca²⁺]_SR, not to alterations in SR volume or Ca²⁺ buffering capacity.