Activation of cardiac sympathetic afferents leads to excitatory cardiovascular reflexes and pain during myocardial ischemia. We hypothesized that cardiac sympathetic afferents are activated by reactive oxygen species produced during ischemia and reperfusion. Single-unit nerve activity of 55 afferents was recorded from the left paravertebral sympathetic chain (T1-T4) in cats anesthetized with alpha-chloralose. Receptive fields of all afferents were located on the right or left ventricle. Mechanical and chemical sensitivities of each afferent ending were evaluated by von Frey hairs, cardiac distension, and local application of bradykinin (BK, 142 pmol) or H2O2 (7.5-15 mumol) to the receptive field. Thirty-one afferents (56%) were responsive to bradykinin (BK), H2O2, and ischemia (2 or 10 min). Deferoxamine (Def, 10-100 mg/kg), dimethylthiourea (DMTU, 10-100 mg/kg), or iron-loaded Def (10 mg/kg) were employed to evaluate the role of H2O2 and hydroxyl radicals (.OH) in activating these afferents (10A delta and 21C fibers) during ischemia and reperfusion. Treatment with the nonspecific scavenger DMTU (n = 10) significantly diminished the increase in discharge activity evoked by ischemia and reperfusion. Treatment with Def also significantly attenuated the responses during ischemia and reperfusion. Thus reactive oxygen species, particularly .OH, activate a group of cardiac sympathetic A delta- and C-fiber afferents during myocardial ischemia and reperfusion and may play an important role in mediating cardiovascular sympathetic reflex responses and/or pain transmission.