Effects elicited by adenosine and substance P on ventricular sensory endings of 14 dorsal root ganglion afferent neurons were studied in situ in anesthetized dogs. Sensory-field application of adenosine (1 microM) increased the activity of these neurons by 179%. Application of a nonspecific adenosine antagonist to epicardial sensory fields suppressed ongoing activity in all 14 neurons by 39%. Application of an A1- or A2-adenosine-receptor antagonist suppressed activity generated by 10 of these neurons by 44 and 59%, respectively. Adenosine applied after A1- or A2-receptor blockade increased activity in 10 neurons by 131 and 145%, respectively, indicating that A1- and A2-receptor effects were not additive. Application of substance P (1 microM) to identified sensory fields increased activity in 12 of these neurons by 169%, whereas application of a substance P-receptor antagonist reduced activity generated by these neurons by 75%. Myocardial ischemia increased activity of nine neurons associated with left ventricular sensory fields by 320%, an effect that was counteracted by the nonspecific adenosine-receptor antagonist. It is concluded that A1- and A2-adenosine receptors, as well as substance P receptors, are present on ventricular epicardial sensory nerve endings of dorsal root ganglion neurons that are tonically active during normal states, becoming further activated during ischemia.