Objective: In heart failure, little information is available as to the Ca2+ release function of sarcoplasmic reticulum (SR), which plays a major role in cardiac contractile function. Here, we assessed the rapid kinetics of drug-induced Ca2+ release from cardiac SR in combination with a measurement of ryanodine binding in heart failure. Methods: The SR vesicles were isolated from dog left ventricular (LV) muscles (normal (N), n=10; pacing induced heart failure (HF), n=10). The time course of SR Ca2+ release was continuously monitored by a stopped-flow apparatus using arsenazoIII as a Ca2+ indicator, and Ca2+ uptake and [3H]ryanodine binding assays were done using a filtration method. Results: The amount of Ca2+ uptake was reduced in HF to 55% of N (P2+ release (P3H]ryanodine binding assay revealed a lower Bmax (pmol/mg) in HF than in N (0.91±0.19 vs. 2.64±0.59, PKd (nM) (0.95±0.29 vs. 0.90±0.11, P=n.s.). The [PL] dependency on the enhancement of [3H]ryanodine binding again showed a shift towards lower [PL] in HF than in N. Conclusions: In pacing-induced heart failure, the Ca2+ releasing function of SR is disturbed, which may result in an intra-cellular Ca2+ transient that was slowed down.