Abnormalities in cardiomyocyte Ca(2+) handling contribute to impaired contractile function in heart failure (HF). Experiments on single ryanodine receptors (RyRs) incorporated into lipid bilayers have indicated that RyRs from failing hearts are more active than those from healthy hearts. Here, we analyzed spontaneous Ca(2+) sparks (brief, localized increased in [Ca(2+)]i) to evaluate RyR cluster activity in situ in a mouse post-myocardial infarction (PMI) model of HF. The cardiac ejection fraction of PMI mice was reduced to ∼30% of that of sham-operated (sham) mice, and their cardiomyocytes were hypertrophied. The [Ca(2+)]i transient amplitude and sarcoplasmic reticulum (SR) Ca(2+) load were decreased in intact PMI cardiomyocytes compared with those from sham mice, and spontaneous Ca(2+) sparks were less frequent, whereas the fractional release and the frequency of Ca(2+) waves were both increased, suggesting higher RyR activity. In permeabilized cardiomyocytes, in which the internal solution can be controlled, Ca(2+) sparks were more frequent in PMI cells (under conditions of similar SR Ca(2+) load), confirming the enhanced RyR activity. However, in intact cells from PMI mice, the Ca(2+) sparks frequency normalized by the SR Ca(2+) load in that cell were reduced compared with those in sham mice, indicating that the cytosolic environment in intact cells contributes to the decrease in Ca(2+) spark frequency. Indeed, using an internal "failing solution" with less ATP (as found in HF), we observed a dramatic decrease in Ca(2+) spark frequency in permeabilized PMI and sham myocytes. In conclusion, our data show that, even if isolated RyR channels show more activity in HF, concomitant alterations in intracellular media composition and SR Ca(2+) load may mask these effects at the Ca(2+) spark level in intact cells. Nonetheless, in this scenario, the probability of arrhythmogenic Ca(2+) waves is enhanced, and they play a potential role in the increase in arrhythmia events in HF patients.