Rationale: Acid-sensing ion channels (ASICs) are Na + channels that are activated by acidic pH. Their expression in cardiac afferents and remarkable sensitivity to small pH changes has made them leading candidates to sense cardiac ischemia. Objective: Four genes encode six different ASIC subunits, however it is not yet clear which of the ASIC subunits contribute to the composition of ASICs in cardiac afferents. Methods and Results: Here, we labeled cardiac afferents using a retrograde tracer dye in mice, which allowed for patch-clamp studies of murine cardiac afferents. We found that a higher percentage of cardiac sensory neurons from the dorsal root ganglia respond to acidic pH and generated larger currents compared to those from the nodose ganglia. The ASIC-like current properties of the cardiac dorsal root ganglia neurons from wild-type mice most closely matched the properties of ASIC2a/3 heteromeric channels. This was supported by studies in ASIC-null mice: acid-evoked currents from ASIC3 −/− cardiac afferents matched the properties of ASIC2a channels, and currents from ASIC2 −/− cardiac afferents matched the properties of ASIC3 channels. Conclusions: We conclude that ASIC2a and -3 are the major ASIC subunits in cardiac dorsal root ganglia neurons and provide potential molecular targets to attenuate chest pain and deleterious reflexes associated with cardiac disease.