In ventricular myocytes, activation of protein kinase A (PKA) by 3'-5' cyclic adenosine monophosphate (cAMP) increases the force of contraction by increasing L-type Ca²⁺ channel currents (I_Ca) and sarcoplasmic reticulum (SR) Ca²⁺ release during excitation–contraction coupling. Cyclic-nucleotide phosphodiesterases (PDEs) comprise a large family of enzymes whose role in the cell is to regulate the spatial and temporal profile of cAMP signals by controlling the degradation of this second messenger. At present, however, the molecular identity and functional roles of the PDEs expressed in ventricular myocytes are incompletely understood. Here, we tested the hypothesis that PDE8A plays a critical role in the modulation of at least one compartment of cAMP and hence PKA activity during β-adrenergic receptor (βAR) activation in ventricular myocytes. Consistent with this hypothesis, we found that PDE8A transcript and protein are expressed in ventricular myocytes. Our data indicate that evoked [Ca²⁺]_i transients and I_Ca increased to a much larger extent in PDE8A null (PDE8A^−/−) than in wild-type (WT) myocytes during β-adrenergic signaling activation. In addition, Ca²⁺ spark activity was higher in PDE8A^−/− than in WT myocytes. Our data indicate that PDE8A is a novel cardiac PDE that controls one or more pools of cAMP implicated in regulation of Ca²⁺ movement through cardiomyocyte.